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Oxidative CO2 reforming of methane over stable and active nickel-based catalysts modified with organic agents
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Oxidative CO2 reforming of methane over stable and active nickel-based catalysts modified with organic agents Baitao Li*, Xueyan Qian, Xiujun Wang* Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
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abstract
Article history:
This study targeted the novel silica-supported nickel-based catalyst (Ni/SiO2) modified by
Received 1 March 2015
organic agents. The synergic modification effect of ethylene glycol (EG) and citric acid (CA)
Received in revised form
on the nickel catalyst was investigated. EG was used to pretreat the silica support and CA
13 April 2015
was used in the impregnation solution to synthesize the nickel based catalysts with
Accepted 17 April 2015
different CA loadings. NiCA-x/SiO2-EG (x: molar ratio of CA/Ni ranging from 0.25 to 1.5)
Available online 13 May 2015
catalysts achieved an excellent stability and higher catalytic activity than the catalysts without EG in oxidative CO2 reforming of methane (CH4/CO2/O2 ¼ 40/20/10, total flow
Keywords:
rate ¼ 60 ml/min, reaction temperature ¼ 750 C, and reaction pressure ¼ 1 atm). EG
Methane reforming
addition modified the surface properties of silica support. The use of CA in the impreg-
Nickel-based catalysts
nation solution had a clear effect on the dispersion of NiO and Ni in the silica matrix. For
Citric acid
the catalysts with the same content of CA, the catalysts with EG modification showed the
Ethylene glycol
synergic effect of EG and CA by improving the chemical interaction between Ni and support, resulting in higher dispersion of nickel. The temperature programmed reduction revealed that the reduction peak shifted to higher temperature with increasing CA loading, which was attributed to the smaller metallic Ni size of the reduced catalysts. The transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy confirmed that the addition of organic additive modified the silica surface and retained the metallic Ni species, and thus preventing the metal aggregation at high reaction temperature. The NiCA-1.5/SiO2-EG catalyst exhibited the highest activity, which was due to the small metallic metal size (4 nm) and the strong interaction between silica support and metal species. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding authors. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China. Tel./fax: þ86 20 87112943. E-mail addresses: [email protected] (B. Li), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.04.104 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction The excessive use of oil by the rapid industrial development and population growth have raised the concern of global warming and the depletion of the finite oil reserves. A demand for new and clean energy has increased rapidly. Compared to traditional fossil fuels, natural gas is in a position of prosperity and development in the long run. As the main component of natural gas, methane can serve as a valuable feedstock. The process of the combination of CO2 reforming with partial oxidation of methane (CRPOM) is of great interest for synthesis gas (syngas, a mixture of hydrogen and carbon monoxide) production, which is a versatile intermediate and can be efficiently converted into a variety of valuable products (e. g. methanol, Fischer-Tropsch fuels) [1,2]. This combined process can be operated thermo-neutrally in an energy-efficient approach and achieve the flexible H2/CO ratios favorable for the downstream application [3,4]. In terms of the catalysts for CRPOM, nickel (Ni) catalysts exhibit highly active, economically available and stable operation [5e8], compared with costly and limited available noble metals (Pt, Ru) [9,10]. CRPOM consists of the oxidizing atmosphere near the catalyst bed inlet and the successive reforming atmosphere in the bed outlet [11e13]. In the oxidizing region, exothermic oxidation reaction proceeds rapidly and thus enhancing the catalyst bed temperature [14,15]. This temperature gradient along the catalyst bed is connected to hot spot formation [16e18]. Moreover, Ni-based catalysts have the problem of the loss of active component, the deactivation caused by sintering (the formation of larger metal particles) [19], carbon deposition (the blocking of the metal surface by the accumulation of carbon on the metal) [20e22] and the changes in the oxidation state of the metal active phase [23e25]. The activity of supported nickel catalysts is depended on the number of active sites that is determined by the metallic nickel particles size, loading amount, dispersion of the metal phase, and reduction degree. Silica (SiO2) is one of the most extensively used supports for the Ni-based catalysts, and its surface properties directly affect the catalytic properties. The concentration and nature of hydroxyl groups (silanols) on a silica surface are critical for the dispersion of the supported metal or metal oxide. Synthesis of highly dispersed nickel catalyst requires a strong interaction between the support and the nickel salt precursor. As for the Ni/SiO2 catalyst, the weak interaction between nickel and silica favors the reduction of nickel precursor and promotes agglomeration of nickel particles, and thus reducing the dispersion of supported nickel and the numbers of active sites [26e28]. Many studies have been conducted to increase the dispersion of supported metal oxide by modifying the impregnation solution or silica support, such as adjusting the hydroxyl groups, using organic metal precursors or adsorbing organic groups [29e31]. The promoting effect of ethanolic solution was first reported over SiO2 supported cobalt catalyst [29]. Compared with aqueous solution, the ethanolic cobalt nitrate solution improved the dispersion of cobalt catalysts and retained a high extent of the cobalt phase reduction. The decreasing of Co3O4 crystallite size was attributed to the presence of ethoxyl groups (SieOeC2H5)
which hindered the sintering of Co3O4 by physically interfering during the thermal decomposition of nitrates [30]. Co catalysts supported on CeO2 was prepared by ethanol impregnation and showed good activity in the steam reforming of ethanol [31]. Although the solvent did not influence the size of the cobalt oxide species, organic solvents could suppress the particle growth during reduction and prevent the segregation of Co0 particles. Compared with ethanol, ethylene glycol (EG) with two hydroxyl groups is expected to effectively adjust the silica surface, and has received great attention as a solvent, reductant, and stabilizer. The precursor solution impregnated with EG has been used for the synthesis of monodisperse nonagglomerated metal particles of cobalt, nickel, copper and precious metals in the micrometer and submicrometer range [32]. The well-dispersed and narrow particle-size distribution platinum (Pt)-based electro-catalysts were obtained without the needing any additional stabilizers [33,34]. On the other hand, citric acid (CA) has been used as the organic chelating additives for catalysts because of its ability to chelate metal species to form stable coordination compounds [35,36]. The use of chelating agents has several implications in the material properties: morphology of active phase, dispersion of metal oxides in the support, and the promotion of highly active phases. NiMo catalysts were prepared with different amount of CA and supported on SBA-15 [37]. The effect of CA addition on the activity and selectivity of these catalysts was correlated with the thermal treatment and the pH of the impregnation solutions [38]. The metalesupport interactions and dispersion of Mo species were dependent on the CA loading [37]. Over NiMoCA/ZrO2eTiO2 catalyst, the Ni:CA molar ratio of 1:2 resulted in the highest activity in hydrodesulfurization of dibenzothiophen [39]. The objective of this study is to develop a stable and active nickel-based catalyst for synthesis gas production in the CRPOM process. EG was used to modify the silica support before impregnation with nickel precursor. CA was used in the impregnation solution as an organic ligand. The physicochemical properties of SiO2 supports and the corresponding Ni/SiO2 catalysts with EG and CA were comprehensively characterized using specific surface area, X-ray diffraction, temperature programmed reduction techniques, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The effects of EG and the molar ratio of CA to nickel nitrate on the surface structure and particle size were elucidated.
Experimental Catalysts preparation Commercially available silica (Nanjing Nanda Surface and Interface Chemical Engineering and Technological Research Centre Co., Ltd.) was used as support. The silica was vacuumed at 120 C for 5 h, followed by modification using ethylene glycol (EG). The evaporation of EG was carried out at 75 C for 30 min in a rotary evaporator (RE-52AA, ShangHaiYaRong Biochemistry Instrument Company) and then the
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silica support was dried at 120 C for 12 h. The obtained material was denoted as SiO2-EG. The supported nickel catalysts were prepared using citric acid (CA, C6H8O7$H2O). Specifically, SiO2-EG was impregnated with the aqueous solution containing appropriate amount of Ni(NO3)2$6H2O and CA. The mixture was sonicated at room temperature for 0.5 h. The paste was dried overnight at 110 C and subsequently calcined in static air atmosphere at 500 C for 3 h with a heating rate of 10 C/min. The catalyst was denoted as NiCA-x/SiO2-EG (where x was the molar ratio of CA/Ni, x ¼ 0.25, 1.0, 1.5). The catalysts NiCA-x/SiO2 (x ¼ 0.25, 1.0, 1.5, 2.0), which used unmodified silica (SiO2) as supports, were synthesized using the same procedure. For comparison of the alcohol effect, the silica support was also modified with ethyl alcohol (EA) and glycerol (GL), and impregnated with the aqueous solution of Ni(NO3)2$6H2O. The obtained catalyst was denoted as Ni/SiO2-EA and Ni/SiO2-GL, respectively. The control Ni/SiO2 catalyst, was prepared with unmodified SiO2 and aqueous solution of Ni(NO3)2$6H2O without CA. The nominal nickel metal loading for all catalysts was kept at 5 wt.%.
Catalytic test The catalytic reaction was performed under atmospheric pressure in a continuous down flow quartz reactor (OD 8 mm, ID 6 mm). A 0.1 g portion of catalyst (particle size between 180 and 250 mm) was placed in the center of the reactor and held by quartz wool plugs. The reaction temperature was monitored by a nickelechromium/nickelesilicon thermocouple covered with a quartz thermocouple-well (ID 1.5 mm) which was located at the outlet of the catalysts bed. The flow rate of each gas was controlled by mass flow controller (D07-7B/ZM, Beijing Sevenstar Electronic Co., Ltd.). The tests were carried out at 750 C with a feeding gas of CH4/CO2/O2 (molar ratio: 40/ 20/10) and a total flow rate of 60 ml/min, corresponding to a space velocity of 3.6 104 ml/h$g. Prior to the activity test, the catalyst was reduced in the flowing hydrogen (60 ml/min) at 750 C for 0.5 h. An ice-cold trap was located between the reactor outlet and the sampling port in order to remove the water in the effluent gas. The gas compositions of reactants and products were analyzed by an online gas chromatograph (GC9800, Shanghai Kechuang Chromatograph Instruments Co., Ltd.) equipped with a thermal conductivity detector (TCD) and a stainless steel column packed with TDX-01. A blank reaction test without catalyst was conducted to confirm that the reactor and thermocouple were not catalytically active.
Catalyst characterization The crystalline phases of the catalysts were characterized using X-ray diffraction (XRD) with a Cu Ka (l ¼ 0.154 nm) radiation source (D8 Advance, Bruker). The X-ray tube worked at 40 kV and 40 mA. The XRD patterns were recorded over the scattering angle of 2q from 10 to 80 with step size of 0.02 at an acquisition time of 0.1 s per step. All the samples were ground to fine powder in an agate mortar before XRD measurements. Crystal phases were identified by Joint Committee on Powder Diffraction Standards (JCPDS) files. The average crystallite particle diameter (D) was obtained from diffraction
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peak broadening by means of the Scherrer equation: D ¼ Kl/b cosq, where K ¼ 0.9 for a sphere-like particle, l ¼ 0.154 nm, q ¼ Bragg angle, b ¼ line broadening (in radians) at half maximum intensity of the diffraction peak. The specific surface area was measured by nitrogen adsorption at liquid nitrogen temperature using Micromeritics TriStar II 3020 and calculated by the BET (BrunauerEmmettTeller) model. Before the measurement, the catalysts were thermally treated under vacuum at 150 C for 6 h to eliminate any water existing in the solid pores. Surface morphology was examined using the transmission electron microscopy (TEM) images (JEM-2100HR, JEOL) at an accelerating voltage of 200 kV. Before the measurement, the samples were dispersed ultrasonically in ethanol and then deposited on a TEM copper grid. Average particle size was calculated using the following equation: d ¼ Snid3i /Snid2i , where ni is the number of the particles with diameter di. The reduction behavior of the calcined catalysts was determined using hydrogen temperature-programmed reduction (TPR) measurement on a chemisorption analyzer (AutoChemII 2920, Micromeritics) equipped with a thermal conductivity detector (TCD). Prior to TPR analysis, the catalysts (150 mg) were degassed at 300 C for 0.5 h in argon (Ar) stream. TPR profile was recorded in 10% H2/Ar gas at 50 ml/ min, and the temperature was linearly raised up to 800 C with a heating rate of 5 C/min. The degrees of reduction were calculated by using a CuO reference compound, assuming 100% reduction of CuO to Cu. Chemical functional groups over the dried samples were recorded by Fourier transform infrared (FT-IR) spectroscopy (Tensor 27, Bruker). The measurements were carried out in the mid-infrared range from 4000 cm1e400 cm1, with 4 cm1 resolution. The catalyst surface analysis was performed using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250, Thermo) equipped with an Al Ka monochromatic source (1486.6 eV) at room temperature in a high-vacuum environment. The binding energy was calibrated using Si 2p photo electron peak at 103.5 eV as a reference.
Results and discussion Catalytic activity of Ni/SiO2 with different modification approaches Effect of alcohol on the catalytic activity of Ni/SiO2 catalyst The Ni/SiO2 catalysts modified with three types of alcohols (EA, EG, and GL) exhibited better catalytic activities than the unmodified Ni/SiO2 catalyst (Fig. 1) under the reaction conditions (CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/min, 750 C, 1 atm). The initial CH4 conversion was 76% for the Ni/SiO2 catalyst, and decreased sharply to 60% after 6 h reaction (Fig. 1A). Although Ni/SiO2-EA showed similar CH4 conversion at the beginning of reaction, it exhibited a slight deactivation during the reaction, with CH4 conversion finally decreasing to 65%. Ni/SiO2-EG and Ni/SiO2-GL performed better than Ni/ SiO2-EA and showed higher initial catalytic activity (85% of CH4 conversion) and better stability. CO2 conversion (Fig. 1B) was lower than CH4 conversion (Fig. 1A), which was
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Fig. 1 e Effect of alcohol on CH4 conversion (A), CO2 conversion (B) and yield of H2 (C) over modified Ni/SiO2 catalysts. --- Ni/SiO2, -B- Ni/SiO2-EA, -V- Ni/SiO2-EG, -*Ni/SiO2-GL. Reaction conditions: CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/min, reaction temperature 750 C.
corresponded with previous results [10]. In addition, the modification of Ni/SiO2 catalysts with EG and GL increased the hydrogen yield (Fig. 1C). These results indicated that EA with insufficient alcoholic hydroxyl groups and carbon source could not effectively improve the methane reforming reaction, while the introduction of more hydroxyl group (EG and GL) into catalyst surface could benefit the tiny and equably dispersed nickel particles that afforded more exposed active sites to the reactant gases [40]. However, GL material was difficult to dry because of its high viscosity and boiling point. Therefore, EG was the best alcohol to deliver the nickel particle over silica supports.
Effect of citric acid (CA) loading on the catalytic activity of Ni/SiO2 catalyst The Ni/SiO2 with CA had higher catalytic activity and stability than the one without CA during methane reforming reaction
Fig. 2 e Effect of CA loading on CH4 conversion (A), CO2 conversion (B) and yield of H2 (C) over NiCA-x/SiO2 catalysts. --- Ni/SiO2, -▫- NiCA-0.25/SiO2, -8- NiCA-1.0/ SiO2, -C- NiCA-1.5/SiO2, -B- NiCA-2.0/SiO2. Reaction conditions: CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/ min, reaction temperature 750 C.
(Fig. 2). There was an optimum CA loading beyond which the increase in CA amount produced the further decrease of the activity parameters. The addition of CA (x ¼ 0.25, 1.0, 1.5) led to higher initial CH4 (Fig. 2A), CO2 (Fig. 2B) conversions as well as yield of hydrogen (Fig. 2C). For NiCA-0.25/SiO2 catalyst, the initial conversion of CH4 increased to 84% (versus 76% for Ni/ SiO2). With increasing CA loading up to CA/Ni ¼ 1.5, CH4 conversion continuously increased to 92%. Contrary to the quick deactivation over CA-free Ni/SiO2 catalyst, NiCA-0.25/ SiO2 exhibited a much lower deactivation rate with the conversion of CH4 decreasing from 84% to 80%. At higher CA loadings, the activity and stability of the catalyst enhanced significantly. For NiCA-1.5/SiO2 catalyst, CH4 conversion was almost constant throughout the whole reaction (~90%). However, further increase CA loading (x ¼ 2.0) lowered the activity (Fig. 2A), which was mainly because the higher concentration
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of CA covered the Ni active sites and blocked the accessibility of the methane. Therefore, the enhanced catalytic activity could be obtained only at the appropriate CA concentration.
enhanced the catalytic activity and stability during the reforming reaction and NiCA-1.5/SiO2-EG showed the highest conversions of CH4 and CO2 and H2 yield.
Synergy modification effect of CA and EG over NiCA-x/SiO2-EG catalysts
Physicochemical properties and morphology studies of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts
The catalytic performance of NiCA-x/SiO2 was further improved by modifying the silica support with EG (Fig. 3). The catalysts modified with CA and EG exhibited the catalytic performance in terms of CH4, CO2 conversions and yield of hydrogen close to the equilibrium (99%, 99% and 50%, respectively). The synergy modification with CA and EG was distinct over the catalyst with lower CA loading (e.g. NiCA0.25/SiO2-EG). NiCA-0.25/SiO2-EG exhibited higher conversions of CH4 (~90%) and CO2 (~88%) and H2 yield (41%) than NiCA-0.25/SiO2 (~84%, 78% and 37%, respectively). However, this synergy modification effect became less evident for those with high CA loadings. NiCA-1.0/SiO2-EG and NiCA-1.5/SiO2EG catalysts exhibited more or less better catalytic performance than the corresponding catalysts without EG. Specifically, NiCA-1.0/SiO2-EG catalyst showed slightly higher CH4 conversion and H2 yield than NiCA-1.0/SiO2. CH4 conversion reached to 91% and remained steady during the reaction. When CA loading increased to CA/Ni ¼ 1.5, NiCA-1.5/SiO2-EG catalyst showed similar CH4 conversion with NiCA-1.5/SiO2, but it exhibited higher CO2 conversion. Moreover, the H2 yield was 42% and remained nearly constant. These results demonstrated that CA combined with EG substantially
The physicochemical properties of Ni/SiO2 catalysts modified with CA and EG were summarized (Table 1). The silica support had a surface area of 370 m2/g, while the surface area slightly decreased to 341 m2/g after impregnation with the aqueous solution of nickel nitrate, which was probably caused by the surface blockage from the metal species. In general, the specific surface area gradually increased with CA content, from 343 to 350 m2/g for NiCA-x/SiO2 and from 352 to 360 m2/g for NiCA-x/SiO2-EG. Previous studies [41] found that CA acted as a template for the pore formation during the solegel process and CA silica composite could be regarded as a structure with a polymeric network of silica gel cross-linked in amorphous CA. The space occupied by free CA remained as pores after the decomposition of free CA. The CA forming hydrogen bonds with silanol increased the specific surface area after the calcination. The XRD patterns revealed that the peak intensities of NiO and Ni were visibly diminished by the modification with CA and EG (Fig. 4). A broad peak between 15 and 35 was present for each catalyst, which was attributed to the amorphous silica. For the calcined catalyst (Fig. 4A), Ni/SiO2 exhibited
Fig. 3 e Effect of ethylene glycol (EG) and citric acid (CA) on the conversion of CH4, CO2 conversion and yield of H2 over NiCAx/SiO2 and NiCA-x/SiO2-EG. --- Ni/SiO2, -▫- NiCA-0.25/SiO2, - - NiCA-0.25/SiO2-EG, -8- NiCA-1.0/SiO2, -B- NiCA-1.0/SiO2EG, -C- NiCA-1.5/SiO2, -;- NiCA-1.5/SiO2-EG. Reaction conditions: CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/min, reaction temperature 750 C.
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Table 1 e Structural properties of Ni/SiO2 catalysts modified with CA and EG. Catalyst
SiO2 Ni/SiO2 NiCA-0.25/SiO2 NiCA-1.0/SiO2 NiCA-1.5/SiO2 NiCA-0.25/SiO2-EG NiCA-1.0/SiO2-EG NiCA-1.5/SiO2-EG a b c d e f
Surface area (m2/g)a
NiO Average particle size (nm)b
dNiO (nm)c
dNi (nm)d
dNi/NiO (nm)e
Degree of reduction (%)f
370 341 343 352 350 352 358 360
e 36.2 9.4 6.8 6.7 8.8 5.7 3.9
e 20.5 9.0 e e e e e
e 20.8 10.5 8.5 6.8 9.2 7.0 4.7
e 29.7 11.0 10.0 7.0 10.6 6.2 4.8
e 100 88 91 86 90 90 84
Samples were calcined at 500 C for 3 h. Calculated by the TEM results. Calculated by Scherrer's formula based on the (2 0 0) diffraction of the calcined catalyst. Calculated by Scherrer's formula based on the (1 1 1) diffraction of the reduced catalyst. Calculated by Scherrer's formula based on the (1 1 1) diffraction for Ni and (200) diffraction for NiO of the spent catalyst. Determined by TPR profile.
Fig. 4 e XRD patterns of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts after calcination at 500 C (A) and reduction at 800 C (B). (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (b2) NiCA-0.25/SiO2-EG, (c1) NiCA-1.0/SiO2, (c2) NiCA-1.0/SiO2-EG, (d1) NiCA-1.5/SiO2, (d2) NiCA-1.5/SiO2-EG.
sharp and strong peaks at 37.3 , 43.3 and 62.9 which were assigned to (1 1 1), (2 0 0), and (2 2 0) diffractions of cubic NiO phase (JCPDS 65-5745), suggesting the formation of larger and more crystalline particles. In contrast to Ni/SiO2 catalyst, the diffraction peaks of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts were so broad and weak that it was difficult to estimate the particle size of NiO by peak broadening. Previous studies reported the positive effect of CA during the synthesis of NiMo catalysts supported on SBA-15 (NiMoCA/SBA-15) [38]. Powder XRD patterns of NiMoCA/SBA-15 catalysts did not show the presence of any NiO crystalline phase, indicating a good dispersion of the deposited metal oxide species. Similar XRD patterns were observed over the reduced catalysts (Fig. 4B). Ni/SiO2 showed one phase attributed to metallic Ni (JCPDS 04-0850), and the XRD peaks were observed
at 44.5 , 51.8 and 76.4 corresponding to (1 1 1), (2 0 0), (2 2 0) planes, respectively, indicating that nickel oxide was successfully reduced to metallic nickel. However, the peak intensity was significantly changed after adding EG and CA. The diffraction peaks of NiCA-x/SiO2 and NiCA-x/SiO2-EG became flat and broad. The crystalline size of metallic Ni (Table 1) calculated by the diffraction peak at 44.5 decreased at higher CA/Ni molar ratio, with the order of NiCA-0.25/SiO2 (10.5 nm) > NiCA-1.0/SiO2 (8.5 nm) > NiCA-1.5/SiO2 (6.8 nm), and NiCA-0.25/SiO2-EG (9.2 nm) > NiCA-1.0/SiO2-EG (7.0 nm) > NiCA-1.5/SiO2-EG (4.7 nm). The XRD results proved that the EG pretreatment of silica supports and the CA addition in the impregnation process substantially reduced the Ni particles size and led to high dispersion on the support surface.
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The TEM measurements of NiCA-x/SiO2 and NiCA-x/SiO2EG catalysts after calcination at 500 C showed the changes in the morphology and metal dispersion induced by EG and CA loading (Fig. 5). A number of large particles ranging from 10 to 50 nm (diameter) were observed in the Ni/SiO2 reference catalyst (Fig. 5a). Two or three NiO particles formed the aggregations and progressed into larger clusters on the silica surface, which was consistent with previous findings that the nickel nitrate precursor used in Ni/SiO2 catalyst resulted in large particle size [42]. The important morphology change was
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observed over NiCA-x/SiO2 catalysts (Fig. 5b1, c1 and d1). The distribution of NiO particles in the silica matrix was homogeneous. With the increase in the CA loading, the particle size distributions became narrow, with 2e16, 3e10 and 3e9 nm for NiCA-0.25/SiO2 (Fig. 5b1), NiCA-1.0/SiO2 (Fig. 5c1) and NiCA-1.5/ SiO2 (Fig. 5d1), respectively. The average particle size gradually decreased from 9.4 nm to 6.7 nm (Table 1), much smaller than Ni/SiO2 catalyst (36 nm). This result showed that CA added in the impregnation solution promoted the particle dispersion and decreased the particle size [37]. The effect of CA was
Fig. 5 e TEM images of the calcined NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts with different CA/Ni molar ratios. (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (b2) NiCA-0.25/SiO2-EG, (c1) NiCA-1.0/SiO2, (c2) NiCA-1.0/SiO2-EG, (d1) NiCA-1.5/SiO2, (d2) NiCA-1.5/SiO2EG.
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ascribed to the formation of complexes (the corresponding citrate) that improved active phase dispersion [37,43]. At the same CA content, the particle sizes of the catalysts modified with EG were further reduced (Table 1), for example, 8.8 nm for NiCA-0.25/SiO2-EG versus 9.4 nm for NiCA-0.25/ SiO2. The catalysts with EG exhibited narrow particle size distributions, with 2e14 nm, 2e8 nm and 1e6 nm for NiCA0.25/SiO2-EG (Fig. 5a2), NiCA-1.0/SiO2-EG (Fig. 5b2), and NiCA1.5/SiO2-EG (Fig. 5c2), respectively. The smallest particles with 3.9 nm and uniformly distribution were obtained over NiCA1.5/SiO2-EG catalyst prepared with the largest CA amount in impregnation solution as well as silica modified with ethylene glycol. The enhancing effect of EG modification was the result of the supported metal species interacting with isolated SiOH that led to the formation of smaller supported metal particles [44].
TPR characterization of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts The reduction activity of various catalysts and the interaction between metal particles and support were determined by TPR (Fig. 6). The unsupported NiO was reduced to metallic Ni with a single reduction peak, with the maximum being centered at 360 C [45,46]. In accordance with the high stability of silica, no reduction of the oxide was observed up to 800 C. Ni/SiO2 catalyst showed a sharp reduction peak at 367 C (Fig. 6a), which could be caused by the reduction of “free state” NiO bearing weak interaction with the support [47,48]. This kind of NiO with large particle size (36 nm) had similar nature to the bulk NiO and could be easily reduced. Consistent with the H2TPR profiles, the crystalline phase of NiO was observed on Ni/ SiO2 (Fig. 4), implying that the interaction between NiO with the SiO2 support was weak and NiO was only dispersed as the
“free state” microcrystal particles on the surface of the support (TEM images in Fig. 5a). The nickel phase formed from these NiO species was prone to migration and aggregation during the reduction and reaction process [49], which might be the main reason that Ni/SiO2 catalyst was easily suffered from deactivation. The reduction peak of NiCA-x/SiO2 shifted to higher temperature. The NiCA-0.25/SiO2 catalyst showed two broad and consecutive reduction peaks (Fig. 6b1): one at ~360 C that was almost the same as Ni/SiO2 and the other at ~ 426 C. The peak width for NiCA-0.25/SiO2 was broad (250e550 C), and it was reasonable to assume that small NiO particles were present but did not reduce rapidly owing to low nucleation rate [50]. When CA/Ni increased to 1.0 (Fig. 6c1) and 1.5 (Fig. 6d1), these two peaks merged and a distinct shift appeared in the position of the reduction peak. The reduction temperature increased to 480 C, which was presumably due to the smaller NiO grains making diffusion effects of the reducing gas more pronounced. The TPR profile revealed that small nickel oxide grains were present on the silica support via the addition of CA, which acted as an organic additives [51] through occasional introduction into the first-sphere of coordination of Ni2þ, or through hydrogen-bonding into the second sphere of coordination. This resulted in an increase of the solution viscosity, so that drying this solution would further increase the viscosity and form an amorphous deposit on the silica surface. The small nickel oxide particles would emerge from the calcination of this supposedly uniform deposit. The silica carrier after pretreatment with EG possessed the notable aspects of the reduction profile. For the EG-containing catalysts, especially for NiCA-0.25/SiO2-EG (Fig. 6b2) and NiCA1.5/SiO2-EG (Fig. 6d2) catalysts, besides the reduction peak at ~ 480 C, a higher reduction peak at ~580 C was observed, which could be assigned to the reduction of small NiO particles and/or NiO species interacting strongly with the support to form a few surface layers of silicate-type compounds [52,53]. The strong interaction between the nickel species and support was favorable to enhance Ni dispersion and form ultra small particles [54]. All the catalysts with CA (NiCA-x/ SiO2 and NiCA-x/SiO2-EG) exhibited lower reduction degree than the reference Ni/SiO2 (Table 1). The difficulty to reduce nickel oxide phases on SiO2 can be assigned to the increasing chemical interactions with the support, which could come from the dissolution of SiO2 upon impregnation with the acidic solution of citric acid.
Surface properties of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts
Fig. 6 e TPR profiles of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts with different CA/Ni molar ratios. (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (c1) NiCA-1.0/SiO2, (d1) NiCA-1.5/SiO2, (b2) NiCA-0.25/SiO2-EG, (c2) NiCA-1.0/SiO2-EG, (d2) NiCA-1.5/ SiO2-EG.
The IR spectra for SiO2 (Fig. 7 curve a0) was consistent with the results from previous infrared spectroscopic studies of silica gel [55e58]. Typical peaks of these spectra were the strong broad absorbance bands associated with the stretching mode of hydrogen-bonded SieOH (3700-3200 cm1) [55] and three bands from SieOeSi vibrations [56], that is, the most intense peak at 1100 cm1, that was assigned to the asymmetric SieOeSi stretching vibration; a peak at 800 cm1, that was assigned to the symmetric stretching vibration, and a peak at 480 cm1 corresponding to the asymmetric bending vibration [57]. The band at 975 cm1 was assigned to a SieO(H)
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apparently related to the ultra small particles and higher dispersion originated from the strong interaction between nickel particles and silica support [62].
Characterization of the spent catalysts Under the reaction condition of CH4/CO2/O2 ¼ 40/20/10, the catalysts were exposed in the oxidized and reforming atmosphere, which would induce the change of chemical state (NiO or Ni). XPS result (Fig. 8) showed Ni2p3/2 peak was centered at 854.2 eV and accompanied by an uptake satellite peak at 860 eV for each sample, which was the typical characteristic peak for Ni0. Compared with the standard binding energy (BE) of Ni0 at 852 ± 0.4 eV [63], all of the Ni 2p BEs of spent catalysts were shifted to higher values. This shift indicated that the interactions between Ni species and support species were significantly enhanced during the reforming process [64]. The XRD patterns of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts after 6-h reaction at 750 C were compared (Fig. 9). Fig. 7 e Infrared spectra of dried NiCA-x/SiO2 and NiCA-x/ SiO2-EG catalysts. (a0) SiO2, (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (b2) NiCA-0.25/SiO2-EG, (c1) NiCA-1.0/SiO2, (c2) NiCA-1.0/ SiO2-EG, (d1) NiCA-1.5/SiO2, (d2) NiCA-1.5/SiO2-EG.
stretching vibration [58]. The peak at 1630 cm1 was the result of the bending vibration of water molecules adsorbed in the samples. After impregnation with nickel nitrate, all the sample (Fig. 7 curves a-d2) exhibited a strong and sharp absorption 1 band from symmetric NO 3 stretch (1360 cm ) [59]. For the EGcontaining catalysts (Fig. 7 curves b2, c2 and d2), a shoulder peak at 880 cm1 [60] was associated with the eOH band from EG. As for the CA-containing catalysts (Fig. 7 curves b1-d2), two other bands were observed at 1560 and 1720 cm1. The former was attributed to asymmetric vibration of COO, while the latter was the characteristic of the protonated COOH groups [51]. The IR bands associated to the symmetric vibrations of carboxylate and carboxylic groups around 1417 cm1 were merged with the broad absorption band from nitrate ions at 1360 cm-1. When CA was added in a higher amount (Fig. 7 curves c1, c2, d1 and d2) the main IR band for CA was at 1720 cm1, implying that the molecule remained protonated. For low CA/Ni2þ molar ratio (curve b1, b2), no protonated COOH group was observed. This IR vibration intensity increased with the CA loading, since CA contains three COOH functional groups and only two of them are needed to react with Ni2þ during the preparation of the solution [51]. For the Ni/SiO2 containing CA or EG, the SieOeSi band (1100, 800, 480 cm1) became strong (Fig. 7), indicating that the CA and EG significantly modified surface properties of silica support. The enhanced SieOeSi band of silica surface increased the negative charge on the pretreated silica surface, strengthened the interaction between supported nickel and silica, and resulted in a higher nickel dispersion [61]. These results proved that the pretreatment of silica support by EG and CA significantly improved the dispersion of the supported Ni. The catalytic activity of NiCA-x/SiO2 and NiCA-x/SiO2-EG were enhanced by the addition of CA and EG, which was
Fig. 8 e The Ni 2p XPS profiles of used NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts with different CA loadings. (A) Ni/ SiO2, (B) NiCA-0.25/SiO2, (C) NiCA-0.25/SiO2-EG, (D) NiCA1.5/SiO2, (E) NiCA-1.5/SiO2-EG.
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Fig. 9 e XRD patterns of used NiCA-x/SiO2 and NiCA-x/ SiO2-EG catalysts with different CA loadings. (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (c1) NiCA-1.0/SiO2, (d1) NiCA-1.5/SiO2, (c2) NiCA-1.0/SiO2-EG, (b2) NiCA-0.25/SiO2-EG, (d2) NiCA-1.5/ SiO2-EG.
Characteristic XRD peak for graphic carbon (2q ¼ 26.6 ) was not observed in all the spent catalysts, indicating no carbon deposition was formed during the reaction. For Ni/SiO2 catalyst, the phase corresponding to Ni was observed, suggesting that NiO was reduced to metallic Ni during the whole reaction process. The diameter of Ni particles over used Ni/SiO2 was larger than the reduced catalyst (29.7 versus 20.8 nm) (Table 1) and the particle distribution was mainly concentrated over the range of 15e30 nm (Fig. 10A), indicating that the sintering of the nickel particle was occurred and accounted for the declination in the catalytic performance. For three NiCA-x/SiO2 catalysts (Fig. 9A), the diffraction peaks assigned to NiO was observed. It could be assumed that CA favored the formation of nickel oxide under the presence of oxygen during the reaction. However, compared with the reduced catalysts, the change in NiO particle size could be negligible (7e11 versus 6.8e10.5 nm) (Table 1) and the NiO did not result in the decline of the catalytic performance. It should be noted that for the NiCA-x/SiO2-EG catalysts (Fig. 9B), along with the increase of the citric acid loading, the metal oxides in the catalysts were gradually reduced to metallic Ni during the reaction process. When x ¼ 0.25, the XRD patterns showed only NiO diffraction peak, with particle size of 10.6 nm (Table 1). With increasing CA loading to x ¼ 1.0 and 1.5, the diffraction peak of Ni appeared and NiO completely disappeared. Most importantly, the nickel particles for these two spent catalysts showed no significantly increased (Table 1). Specifically, the
Fig. 10 e TEM images of the spent Ni/SiO2 (A) and NiCA-1.5/ SiO2-EG (B) catalysts.
distribution of nickel was homogenous and located at 2e5 nm (Fig. 10B), suggesting that the sintering and oxidation of Ni species during the reaction were effectively suppressed by the co-existence of EG and higher loading of CA. This synergic effect can enhance the interaction between metal and carrier so as to prevent metal-oxide from oxidation and sintering. Since the EG and CA modification is feasible to conduct in the realworld application, the NiCA-x/SiO2-EG catalysts tested in this study exhibited a great potential to produce syngas in the oxidative CO2 reforming of methane.
Conclusion The catalytic activity of Ni/SiO2 catalysts modified with organic agents (CA and EG) for the oxidative CO2 reforming of methane was comprehensively investigated in this study. There were three major conclusions: First, the best alcoholic effect was reflected by EG which was due to the supported metal species interacting with strong SiO-Si group, leading to the formation of smaller metal particles. Second, the appropriate CA concentration in the impregnation solution was favorable for retaining the metallic nickel and preventing the aggregation of the nickel particles in the methane reforming involved with oxygen. The results demonstrated a simple way to produce small nickel particle size (about 4 nm) with high metal dispersion by modification Ni/SiO2 with CA and EG. Third, the synergic modification effect of CA and EG over NiCA-x/SiO2-EG catalysts showed higher catalytic performance and better stability than NiCA-x/SiO2 catalyst.
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NiCA-1.5/SiO2-EG with nickel particles of 4 nm reflected the best catalytic activity and stability. [14]
Acknowledgment The authors are grateful to the financial supports from the National Natural Science Foundation of China (project No. 21173086 and U1301245) and Guangdong Natural Science Foundation (Project No. 2014A030313259).
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Oxidative CO2 reforming of methane over stable and active nickel-based catalysts modified with organic agents Baitao Li*, Xueyan Qian, Xiujun Wang* Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
article info
abstract
Article history:
This study targeted the novel silica-supported nickel-based catalyst (Ni/SiO2) modified by
Received 1 March 2015
organic agents. The synergic modification effect of ethylene glycol (EG) and citric acid (CA)
Received in revised form
on the nickel catalyst was investigated. EG was used to pretreat the silica support and CA
13 April 2015
was used in the impregnation solution to synthesize the nickel based catalysts with
Accepted 17 April 2015
different CA loadings. NiCA-x/SiO2-EG (x: molar ratio of CA/Ni ranging from 0.25 to 1.5)
Available online 13 May 2015
catalysts achieved an excellent stability and higher catalytic activity than the catalysts without EG in oxidative CO2 reforming of methane (CH4/CO2/O2 ¼ 40/20/10, total flow
Keywords:
rate ¼ 60 ml/min, reaction temperature ¼ 750 C, and reaction pressure ¼ 1 atm). EG
Methane reforming
addition modified the surface properties of silica support. The use of CA in the impreg-
Nickel-based catalysts
nation solution had a clear effect on the dispersion of NiO and Ni in the silica matrix. For
Citric acid
the catalysts with the same content of CA, the catalysts with EG modification showed the
Ethylene glycol
synergic effect of EG and CA by improving the chemical interaction between Ni and support, resulting in higher dispersion of nickel. The temperature programmed reduction revealed that the reduction peak shifted to higher temperature with increasing CA loading, which was attributed to the smaller metallic Ni size of the reduced catalysts. The transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy confirmed that the addition of organic additive modified the silica surface and retained the metallic Ni species, and thus preventing the metal aggregation at high reaction temperature. The NiCA-1.5/SiO2-EG catalyst exhibited the highest activity, which was due to the small metallic metal size (4 nm) and the strong interaction between silica support and metal species. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding authors. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China. Tel./fax: þ86 20 87112943. E-mail addresses: [email protected] (B. Li), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.04.104 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction The excessive use of oil by the rapid industrial development and population growth have raised the concern of global warming and the depletion of the finite oil reserves. A demand for new and clean energy has increased rapidly. Compared to traditional fossil fuels, natural gas is in a position of prosperity and development in the long run. As the main component of natural gas, methane can serve as a valuable feedstock. The process of the combination of CO2 reforming with partial oxidation of methane (CRPOM) is of great interest for synthesis gas (syngas, a mixture of hydrogen and carbon monoxide) production, which is a versatile intermediate and can be efficiently converted into a variety of valuable products (e. g. methanol, Fischer-Tropsch fuels) [1,2]. This combined process can be operated thermo-neutrally in an energy-efficient approach and achieve the flexible H2/CO ratios favorable for the downstream application [3,4]. In terms of the catalysts for CRPOM, nickel (Ni) catalysts exhibit highly active, economically available and stable operation [5e8], compared with costly and limited available noble metals (Pt, Ru) [9,10]. CRPOM consists of the oxidizing atmosphere near the catalyst bed inlet and the successive reforming atmosphere in the bed outlet [11e13]. In the oxidizing region, exothermic oxidation reaction proceeds rapidly and thus enhancing the catalyst bed temperature [14,15]. This temperature gradient along the catalyst bed is connected to hot spot formation [16e18]. Moreover, Ni-based catalysts have the problem of the loss of active component, the deactivation caused by sintering (the formation of larger metal particles) [19], carbon deposition (the blocking of the metal surface by the accumulation of carbon on the metal) [20e22] and the changes in the oxidation state of the metal active phase [23e25]. The activity of supported nickel catalysts is depended on the number of active sites that is determined by the metallic nickel particles size, loading amount, dispersion of the metal phase, and reduction degree. Silica (SiO2) is one of the most extensively used supports for the Ni-based catalysts, and its surface properties directly affect the catalytic properties. The concentration and nature of hydroxyl groups (silanols) on a silica surface are critical for the dispersion of the supported metal or metal oxide. Synthesis of highly dispersed nickel catalyst requires a strong interaction between the support and the nickel salt precursor. As for the Ni/SiO2 catalyst, the weak interaction between nickel and silica favors the reduction of nickel precursor and promotes agglomeration of nickel particles, and thus reducing the dispersion of supported nickel and the numbers of active sites [26e28]. Many studies have been conducted to increase the dispersion of supported metal oxide by modifying the impregnation solution or silica support, such as adjusting the hydroxyl groups, using organic metal precursors or adsorbing organic groups [29e31]. The promoting effect of ethanolic solution was first reported over SiO2 supported cobalt catalyst [29]. Compared with aqueous solution, the ethanolic cobalt nitrate solution improved the dispersion of cobalt catalysts and retained a high extent of the cobalt phase reduction. The decreasing of Co3O4 crystallite size was attributed to the presence of ethoxyl groups (SieOeC2H5)
which hindered the sintering of Co3O4 by physically interfering during the thermal decomposition of nitrates [30]. Co catalysts supported on CeO2 was prepared by ethanol impregnation and showed good activity in the steam reforming of ethanol [31]. Although the solvent did not influence the size of the cobalt oxide species, organic solvents could suppress the particle growth during reduction and prevent the segregation of Co0 particles. Compared with ethanol, ethylene glycol (EG) with two hydroxyl groups is expected to effectively adjust the silica surface, and has received great attention as a solvent, reductant, and stabilizer. The precursor solution impregnated with EG has been used for the synthesis of monodisperse nonagglomerated metal particles of cobalt, nickel, copper and precious metals in the micrometer and submicrometer range [32]. The well-dispersed and narrow particle-size distribution platinum (Pt)-based electro-catalysts were obtained without the needing any additional stabilizers [33,34]. On the other hand, citric acid (CA) has been used as the organic chelating additives for catalysts because of its ability to chelate metal species to form stable coordination compounds [35,36]. The use of chelating agents has several implications in the material properties: morphology of active phase, dispersion of metal oxides in the support, and the promotion of highly active phases. NiMo catalysts were prepared with different amount of CA and supported on SBA-15 [37]. The effect of CA addition on the activity and selectivity of these catalysts was correlated with the thermal treatment and the pH of the impregnation solutions [38]. The metalesupport interactions and dispersion of Mo species were dependent on the CA loading [37]. Over NiMoCA/ZrO2eTiO2 catalyst, the Ni:CA molar ratio of 1:2 resulted in the highest activity in hydrodesulfurization of dibenzothiophen [39]. The objective of this study is to develop a stable and active nickel-based catalyst for synthesis gas production in the CRPOM process. EG was used to modify the silica support before impregnation with nickel precursor. CA was used in the impregnation solution as an organic ligand. The physicochemical properties of SiO2 supports and the corresponding Ni/SiO2 catalysts with EG and CA were comprehensively characterized using specific surface area, X-ray diffraction, temperature programmed reduction techniques, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The effects of EG and the molar ratio of CA to nickel nitrate on the surface structure and particle size were elucidated.
Experimental Catalysts preparation Commercially available silica (Nanjing Nanda Surface and Interface Chemical Engineering and Technological Research Centre Co., Ltd.) was used as support. The silica was vacuumed at 120 C for 5 h, followed by modification using ethylene glycol (EG). The evaporation of EG was carried out at 75 C for 30 min in a rotary evaporator (RE-52AA, ShangHaiYaRong Biochemistry Instrument Company) and then the
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silica support was dried at 120 C for 12 h. The obtained material was denoted as SiO2-EG. The supported nickel catalysts were prepared using citric acid (CA, C6H8O7$H2O). Specifically, SiO2-EG was impregnated with the aqueous solution containing appropriate amount of Ni(NO3)2$6H2O and CA. The mixture was sonicated at room temperature for 0.5 h. The paste was dried overnight at 110 C and subsequently calcined in static air atmosphere at 500 C for 3 h with a heating rate of 10 C/min. The catalyst was denoted as NiCA-x/SiO2-EG (where x was the molar ratio of CA/Ni, x ¼ 0.25, 1.0, 1.5). The catalysts NiCA-x/SiO2 (x ¼ 0.25, 1.0, 1.5, 2.0), which used unmodified silica (SiO2) as supports, were synthesized using the same procedure. For comparison of the alcohol effect, the silica support was also modified with ethyl alcohol (EA) and glycerol (GL), and impregnated with the aqueous solution of Ni(NO3)2$6H2O. The obtained catalyst was denoted as Ni/SiO2-EA and Ni/SiO2-GL, respectively. The control Ni/SiO2 catalyst, was prepared with unmodified SiO2 and aqueous solution of Ni(NO3)2$6H2O without CA. The nominal nickel metal loading for all catalysts was kept at 5 wt.%.
Catalytic test The catalytic reaction was performed under atmospheric pressure in a continuous down flow quartz reactor (OD 8 mm, ID 6 mm). A 0.1 g portion of catalyst (particle size between 180 and 250 mm) was placed in the center of the reactor and held by quartz wool plugs. The reaction temperature was monitored by a nickelechromium/nickelesilicon thermocouple covered with a quartz thermocouple-well (ID 1.5 mm) which was located at the outlet of the catalysts bed. The flow rate of each gas was controlled by mass flow controller (D07-7B/ZM, Beijing Sevenstar Electronic Co., Ltd.). The tests were carried out at 750 C with a feeding gas of CH4/CO2/O2 (molar ratio: 40/ 20/10) and a total flow rate of 60 ml/min, corresponding to a space velocity of 3.6 104 ml/h$g. Prior to the activity test, the catalyst was reduced in the flowing hydrogen (60 ml/min) at 750 C for 0.5 h. An ice-cold trap was located between the reactor outlet and the sampling port in order to remove the water in the effluent gas. The gas compositions of reactants and products were analyzed by an online gas chromatograph (GC9800, Shanghai Kechuang Chromatograph Instruments Co., Ltd.) equipped with a thermal conductivity detector (TCD) and a stainless steel column packed with TDX-01. A blank reaction test without catalyst was conducted to confirm that the reactor and thermocouple were not catalytically active.
Catalyst characterization The crystalline phases of the catalysts were characterized using X-ray diffraction (XRD) with a Cu Ka (l ¼ 0.154 nm) radiation source (D8 Advance, Bruker). The X-ray tube worked at 40 kV and 40 mA. The XRD patterns were recorded over the scattering angle of 2q from 10 to 80 with step size of 0.02 at an acquisition time of 0.1 s per step. All the samples were ground to fine powder in an agate mortar before XRD measurements. Crystal phases were identified by Joint Committee on Powder Diffraction Standards (JCPDS) files. The average crystallite particle diameter (D) was obtained from diffraction
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peak broadening by means of the Scherrer equation: D ¼ Kl/b cosq, where K ¼ 0.9 for a sphere-like particle, l ¼ 0.154 nm, q ¼ Bragg angle, b ¼ line broadening (in radians) at half maximum intensity of the diffraction peak. The specific surface area was measured by nitrogen adsorption at liquid nitrogen temperature using Micromeritics TriStar II 3020 and calculated by the BET (BrunauerEmmettTeller) model. Before the measurement, the catalysts were thermally treated under vacuum at 150 C for 6 h to eliminate any water existing in the solid pores. Surface morphology was examined using the transmission electron microscopy (TEM) images (JEM-2100HR, JEOL) at an accelerating voltage of 200 kV. Before the measurement, the samples were dispersed ultrasonically in ethanol and then deposited on a TEM copper grid. Average particle size was calculated using the following equation: d ¼ Snid3i /Snid2i , where ni is the number of the particles with diameter di. The reduction behavior of the calcined catalysts was determined using hydrogen temperature-programmed reduction (TPR) measurement on a chemisorption analyzer (AutoChemII 2920, Micromeritics) equipped with a thermal conductivity detector (TCD). Prior to TPR analysis, the catalysts (150 mg) were degassed at 300 C for 0.5 h in argon (Ar) stream. TPR profile was recorded in 10% H2/Ar gas at 50 ml/ min, and the temperature was linearly raised up to 800 C with a heating rate of 5 C/min. The degrees of reduction were calculated by using a CuO reference compound, assuming 100% reduction of CuO to Cu. Chemical functional groups over the dried samples were recorded by Fourier transform infrared (FT-IR) spectroscopy (Tensor 27, Bruker). The measurements were carried out in the mid-infrared range from 4000 cm1e400 cm1, with 4 cm1 resolution. The catalyst surface analysis was performed using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250, Thermo) equipped with an Al Ka monochromatic source (1486.6 eV) at room temperature in a high-vacuum environment. The binding energy was calibrated using Si 2p photo electron peak at 103.5 eV as a reference.
Results and discussion Catalytic activity of Ni/SiO2 with different modification approaches Effect of alcohol on the catalytic activity of Ni/SiO2 catalyst The Ni/SiO2 catalysts modified with three types of alcohols (EA, EG, and GL) exhibited better catalytic activities than the unmodified Ni/SiO2 catalyst (Fig. 1) under the reaction conditions (CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/min, 750 C, 1 atm). The initial CH4 conversion was 76% for the Ni/SiO2 catalyst, and decreased sharply to 60% after 6 h reaction (Fig. 1A). Although Ni/SiO2-EA showed similar CH4 conversion at the beginning of reaction, it exhibited a slight deactivation during the reaction, with CH4 conversion finally decreasing to 65%. Ni/SiO2-EG and Ni/SiO2-GL performed better than Ni/ SiO2-EA and showed higher initial catalytic activity (85% of CH4 conversion) and better stability. CO2 conversion (Fig. 1B) was lower than CH4 conversion (Fig. 1A), which was
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Fig. 1 e Effect of alcohol on CH4 conversion (A), CO2 conversion (B) and yield of H2 (C) over modified Ni/SiO2 catalysts. --- Ni/SiO2, -B- Ni/SiO2-EA, -V- Ni/SiO2-EG, -*Ni/SiO2-GL. Reaction conditions: CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/min, reaction temperature 750 C.
corresponded with previous results [10]. In addition, the modification of Ni/SiO2 catalysts with EG and GL increased the hydrogen yield (Fig. 1C). These results indicated that EA with insufficient alcoholic hydroxyl groups and carbon source could not effectively improve the methane reforming reaction, while the introduction of more hydroxyl group (EG and GL) into catalyst surface could benefit the tiny and equably dispersed nickel particles that afforded more exposed active sites to the reactant gases [40]. However, GL material was difficult to dry because of its high viscosity and boiling point. Therefore, EG was the best alcohol to deliver the nickel particle over silica supports.
Effect of citric acid (CA) loading on the catalytic activity of Ni/SiO2 catalyst The Ni/SiO2 with CA had higher catalytic activity and stability than the one without CA during methane reforming reaction
Fig. 2 e Effect of CA loading on CH4 conversion (A), CO2 conversion (B) and yield of H2 (C) over NiCA-x/SiO2 catalysts. --- Ni/SiO2, -▫- NiCA-0.25/SiO2, -8- NiCA-1.0/ SiO2, -C- NiCA-1.5/SiO2, -B- NiCA-2.0/SiO2. Reaction conditions: CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/ min, reaction temperature 750 C.
(Fig. 2). There was an optimum CA loading beyond which the increase in CA amount produced the further decrease of the activity parameters. The addition of CA (x ¼ 0.25, 1.0, 1.5) led to higher initial CH4 (Fig. 2A), CO2 (Fig. 2B) conversions as well as yield of hydrogen (Fig. 2C). For NiCA-0.25/SiO2 catalyst, the initial conversion of CH4 increased to 84% (versus 76% for Ni/ SiO2). With increasing CA loading up to CA/Ni ¼ 1.5, CH4 conversion continuously increased to 92%. Contrary to the quick deactivation over CA-free Ni/SiO2 catalyst, NiCA-0.25/ SiO2 exhibited a much lower deactivation rate with the conversion of CH4 decreasing from 84% to 80%. At higher CA loadings, the activity and stability of the catalyst enhanced significantly. For NiCA-1.5/SiO2 catalyst, CH4 conversion was almost constant throughout the whole reaction (~90%). However, further increase CA loading (x ¼ 2.0) lowered the activity (Fig. 2A), which was mainly because the higher concentration
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of CA covered the Ni active sites and blocked the accessibility of the methane. Therefore, the enhanced catalytic activity could be obtained only at the appropriate CA concentration.
enhanced the catalytic activity and stability during the reforming reaction and NiCA-1.5/SiO2-EG showed the highest conversions of CH4 and CO2 and H2 yield.
Synergy modification effect of CA and EG over NiCA-x/SiO2-EG catalysts
Physicochemical properties and morphology studies of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts
The catalytic performance of NiCA-x/SiO2 was further improved by modifying the silica support with EG (Fig. 3). The catalysts modified with CA and EG exhibited the catalytic performance in terms of CH4, CO2 conversions and yield of hydrogen close to the equilibrium (99%, 99% and 50%, respectively). The synergy modification with CA and EG was distinct over the catalyst with lower CA loading (e.g. NiCA0.25/SiO2-EG). NiCA-0.25/SiO2-EG exhibited higher conversions of CH4 (~90%) and CO2 (~88%) and H2 yield (41%) than NiCA-0.25/SiO2 (~84%, 78% and 37%, respectively). However, this synergy modification effect became less evident for those with high CA loadings. NiCA-1.0/SiO2-EG and NiCA-1.5/SiO2EG catalysts exhibited more or less better catalytic performance than the corresponding catalysts without EG. Specifically, NiCA-1.0/SiO2-EG catalyst showed slightly higher CH4 conversion and H2 yield than NiCA-1.0/SiO2. CH4 conversion reached to 91% and remained steady during the reaction. When CA loading increased to CA/Ni ¼ 1.5, NiCA-1.5/SiO2-EG catalyst showed similar CH4 conversion with NiCA-1.5/SiO2, but it exhibited higher CO2 conversion. Moreover, the H2 yield was 42% and remained nearly constant. These results demonstrated that CA combined with EG substantially
The physicochemical properties of Ni/SiO2 catalysts modified with CA and EG were summarized (Table 1). The silica support had a surface area of 370 m2/g, while the surface area slightly decreased to 341 m2/g after impregnation with the aqueous solution of nickel nitrate, which was probably caused by the surface blockage from the metal species. In general, the specific surface area gradually increased with CA content, from 343 to 350 m2/g for NiCA-x/SiO2 and from 352 to 360 m2/g for NiCA-x/SiO2-EG. Previous studies [41] found that CA acted as a template for the pore formation during the solegel process and CA silica composite could be regarded as a structure with a polymeric network of silica gel cross-linked in amorphous CA. The space occupied by free CA remained as pores after the decomposition of free CA. The CA forming hydrogen bonds with silanol increased the specific surface area after the calcination. The XRD patterns revealed that the peak intensities of NiO and Ni were visibly diminished by the modification with CA and EG (Fig. 4). A broad peak between 15 and 35 was present for each catalyst, which was attributed to the amorphous silica. For the calcined catalyst (Fig. 4A), Ni/SiO2 exhibited
Fig. 3 e Effect of ethylene glycol (EG) and citric acid (CA) on the conversion of CH4, CO2 conversion and yield of H2 over NiCAx/SiO2 and NiCA-x/SiO2-EG. --- Ni/SiO2, -▫- NiCA-0.25/SiO2, - - NiCA-0.25/SiO2-EG, -8- NiCA-1.0/SiO2, -B- NiCA-1.0/SiO2EG, -C- NiCA-1.5/SiO2, -;- NiCA-1.5/SiO2-EG. Reaction conditions: CH4/CO2/O2 ¼ 40/20/10, total flow rate 60 ml/min, reaction temperature 750 C.
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Table 1 e Structural properties of Ni/SiO2 catalysts modified with CA and EG. Catalyst
SiO2 Ni/SiO2 NiCA-0.25/SiO2 NiCA-1.0/SiO2 NiCA-1.5/SiO2 NiCA-0.25/SiO2-EG NiCA-1.0/SiO2-EG NiCA-1.5/SiO2-EG a b c d e f
Surface area (m2/g)a
NiO Average particle size (nm)b
dNiO (nm)c
dNi (nm)d
dNi/NiO (nm)e
Degree of reduction (%)f
370 341 343 352 350 352 358 360
e 36.2 9.4 6.8 6.7 8.8 5.7 3.9
e 20.5 9.0 e e e e e
e 20.8 10.5 8.5 6.8 9.2 7.0 4.7
e 29.7 11.0 10.0 7.0 10.6 6.2 4.8
e 100 88 91 86 90 90 84
Samples were calcined at 500 C for 3 h. Calculated by the TEM results. Calculated by Scherrer's formula based on the (2 0 0) diffraction of the calcined catalyst. Calculated by Scherrer's formula based on the (1 1 1) diffraction of the reduced catalyst. Calculated by Scherrer's formula based on the (1 1 1) diffraction for Ni and (200) diffraction for NiO of the spent catalyst. Determined by TPR profile.
Fig. 4 e XRD patterns of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts after calcination at 500 C (A) and reduction at 800 C (B). (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (b2) NiCA-0.25/SiO2-EG, (c1) NiCA-1.0/SiO2, (c2) NiCA-1.0/SiO2-EG, (d1) NiCA-1.5/SiO2, (d2) NiCA-1.5/SiO2-EG.
sharp and strong peaks at 37.3 , 43.3 and 62.9 which were assigned to (1 1 1), (2 0 0), and (2 2 0) diffractions of cubic NiO phase (JCPDS 65-5745), suggesting the formation of larger and more crystalline particles. In contrast to Ni/SiO2 catalyst, the diffraction peaks of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts were so broad and weak that it was difficult to estimate the particle size of NiO by peak broadening. Previous studies reported the positive effect of CA during the synthesis of NiMo catalysts supported on SBA-15 (NiMoCA/SBA-15) [38]. Powder XRD patterns of NiMoCA/SBA-15 catalysts did not show the presence of any NiO crystalline phase, indicating a good dispersion of the deposited metal oxide species. Similar XRD patterns were observed over the reduced catalysts (Fig. 4B). Ni/SiO2 showed one phase attributed to metallic Ni (JCPDS 04-0850), and the XRD peaks were observed
at 44.5 , 51.8 and 76.4 corresponding to (1 1 1), (2 0 0), (2 2 0) planes, respectively, indicating that nickel oxide was successfully reduced to metallic nickel. However, the peak intensity was significantly changed after adding EG and CA. The diffraction peaks of NiCA-x/SiO2 and NiCA-x/SiO2-EG became flat and broad. The crystalline size of metallic Ni (Table 1) calculated by the diffraction peak at 44.5 decreased at higher CA/Ni molar ratio, with the order of NiCA-0.25/SiO2 (10.5 nm) > NiCA-1.0/SiO2 (8.5 nm) > NiCA-1.5/SiO2 (6.8 nm), and NiCA-0.25/SiO2-EG (9.2 nm) > NiCA-1.0/SiO2-EG (7.0 nm) > NiCA-1.5/SiO2-EG (4.7 nm). The XRD results proved that the EG pretreatment of silica supports and the CA addition in the impregnation process substantially reduced the Ni particles size and led to high dispersion on the support surface.
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The TEM measurements of NiCA-x/SiO2 and NiCA-x/SiO2EG catalysts after calcination at 500 C showed the changes in the morphology and metal dispersion induced by EG and CA loading (Fig. 5). A number of large particles ranging from 10 to 50 nm (diameter) were observed in the Ni/SiO2 reference catalyst (Fig. 5a). Two or three NiO particles formed the aggregations and progressed into larger clusters on the silica surface, which was consistent with previous findings that the nickel nitrate precursor used in Ni/SiO2 catalyst resulted in large particle size [42]. The important morphology change was
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observed over NiCA-x/SiO2 catalysts (Fig. 5b1, c1 and d1). The distribution of NiO particles in the silica matrix was homogeneous. With the increase in the CA loading, the particle size distributions became narrow, with 2e16, 3e10 and 3e9 nm for NiCA-0.25/SiO2 (Fig. 5b1), NiCA-1.0/SiO2 (Fig. 5c1) and NiCA-1.5/ SiO2 (Fig. 5d1), respectively. The average particle size gradually decreased from 9.4 nm to 6.7 nm (Table 1), much smaller than Ni/SiO2 catalyst (36 nm). This result showed that CA added in the impregnation solution promoted the particle dispersion and decreased the particle size [37]. The effect of CA was
Fig. 5 e TEM images of the calcined NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts with different CA/Ni molar ratios. (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (b2) NiCA-0.25/SiO2-EG, (c1) NiCA-1.0/SiO2, (c2) NiCA-1.0/SiO2-EG, (d1) NiCA-1.5/SiO2, (d2) NiCA-1.5/SiO2EG.
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ascribed to the formation of complexes (the corresponding citrate) that improved active phase dispersion [37,43]. At the same CA content, the particle sizes of the catalysts modified with EG were further reduced (Table 1), for example, 8.8 nm for NiCA-0.25/SiO2-EG versus 9.4 nm for NiCA-0.25/ SiO2. The catalysts with EG exhibited narrow particle size distributions, with 2e14 nm, 2e8 nm and 1e6 nm for NiCA0.25/SiO2-EG (Fig. 5a2), NiCA-1.0/SiO2-EG (Fig. 5b2), and NiCA1.5/SiO2-EG (Fig. 5c2), respectively. The smallest particles with 3.9 nm and uniformly distribution were obtained over NiCA1.5/SiO2-EG catalyst prepared with the largest CA amount in impregnation solution as well as silica modified with ethylene glycol. The enhancing effect of EG modification was the result of the supported metal species interacting with isolated SiOH that led to the formation of smaller supported metal particles [44].
TPR characterization of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts The reduction activity of various catalysts and the interaction between metal particles and support were determined by TPR (Fig. 6). The unsupported NiO was reduced to metallic Ni with a single reduction peak, with the maximum being centered at 360 C [45,46]. In accordance with the high stability of silica, no reduction of the oxide was observed up to 800 C. Ni/SiO2 catalyst showed a sharp reduction peak at 367 C (Fig. 6a), which could be caused by the reduction of “free state” NiO bearing weak interaction with the support [47,48]. This kind of NiO with large particle size (36 nm) had similar nature to the bulk NiO and could be easily reduced. Consistent with the H2TPR profiles, the crystalline phase of NiO was observed on Ni/ SiO2 (Fig. 4), implying that the interaction between NiO with the SiO2 support was weak and NiO was only dispersed as the
“free state” microcrystal particles on the surface of the support (TEM images in Fig. 5a). The nickel phase formed from these NiO species was prone to migration and aggregation during the reduction and reaction process [49], which might be the main reason that Ni/SiO2 catalyst was easily suffered from deactivation. The reduction peak of NiCA-x/SiO2 shifted to higher temperature. The NiCA-0.25/SiO2 catalyst showed two broad and consecutive reduction peaks (Fig. 6b1): one at ~360 C that was almost the same as Ni/SiO2 and the other at ~ 426 C. The peak width for NiCA-0.25/SiO2 was broad (250e550 C), and it was reasonable to assume that small NiO particles were present but did not reduce rapidly owing to low nucleation rate [50]. When CA/Ni increased to 1.0 (Fig. 6c1) and 1.5 (Fig. 6d1), these two peaks merged and a distinct shift appeared in the position of the reduction peak. The reduction temperature increased to 480 C, which was presumably due to the smaller NiO grains making diffusion effects of the reducing gas more pronounced. The TPR profile revealed that small nickel oxide grains were present on the silica support via the addition of CA, which acted as an organic additives [51] through occasional introduction into the first-sphere of coordination of Ni2þ, or through hydrogen-bonding into the second sphere of coordination. This resulted in an increase of the solution viscosity, so that drying this solution would further increase the viscosity and form an amorphous deposit on the silica surface. The small nickel oxide particles would emerge from the calcination of this supposedly uniform deposit. The silica carrier after pretreatment with EG possessed the notable aspects of the reduction profile. For the EG-containing catalysts, especially for NiCA-0.25/SiO2-EG (Fig. 6b2) and NiCA1.5/SiO2-EG (Fig. 6d2) catalysts, besides the reduction peak at ~ 480 C, a higher reduction peak at ~580 C was observed, which could be assigned to the reduction of small NiO particles and/or NiO species interacting strongly with the support to form a few surface layers of silicate-type compounds [52,53]. The strong interaction between the nickel species and support was favorable to enhance Ni dispersion and form ultra small particles [54]. All the catalysts with CA (NiCA-x/ SiO2 and NiCA-x/SiO2-EG) exhibited lower reduction degree than the reference Ni/SiO2 (Table 1). The difficulty to reduce nickel oxide phases on SiO2 can be assigned to the increasing chemical interactions with the support, which could come from the dissolution of SiO2 upon impregnation with the acidic solution of citric acid.
Surface properties of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts
Fig. 6 e TPR profiles of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts with different CA/Ni molar ratios. (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (c1) NiCA-1.0/SiO2, (d1) NiCA-1.5/SiO2, (b2) NiCA-0.25/SiO2-EG, (c2) NiCA-1.0/SiO2-EG, (d2) NiCA-1.5/ SiO2-EG.
The IR spectra for SiO2 (Fig. 7 curve a0) was consistent with the results from previous infrared spectroscopic studies of silica gel [55e58]. Typical peaks of these spectra were the strong broad absorbance bands associated with the stretching mode of hydrogen-bonded SieOH (3700-3200 cm1) [55] and three bands from SieOeSi vibrations [56], that is, the most intense peak at 1100 cm1, that was assigned to the asymmetric SieOeSi stretching vibration; a peak at 800 cm1, that was assigned to the symmetric stretching vibration, and a peak at 480 cm1 corresponding to the asymmetric bending vibration [57]. The band at 975 cm1 was assigned to a SieO(H)
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apparently related to the ultra small particles and higher dispersion originated from the strong interaction between nickel particles and silica support [62].
Characterization of the spent catalysts Under the reaction condition of CH4/CO2/O2 ¼ 40/20/10, the catalysts were exposed in the oxidized and reforming atmosphere, which would induce the change of chemical state (NiO or Ni). XPS result (Fig. 8) showed Ni2p3/2 peak was centered at 854.2 eV and accompanied by an uptake satellite peak at 860 eV for each sample, which was the typical characteristic peak for Ni0. Compared with the standard binding energy (BE) of Ni0 at 852 ± 0.4 eV [63], all of the Ni 2p BEs of spent catalysts were shifted to higher values. This shift indicated that the interactions between Ni species and support species were significantly enhanced during the reforming process [64]. The XRD patterns of NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts after 6-h reaction at 750 C were compared (Fig. 9). Fig. 7 e Infrared spectra of dried NiCA-x/SiO2 and NiCA-x/ SiO2-EG catalysts. (a0) SiO2, (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (b2) NiCA-0.25/SiO2-EG, (c1) NiCA-1.0/SiO2, (c2) NiCA-1.0/ SiO2-EG, (d1) NiCA-1.5/SiO2, (d2) NiCA-1.5/SiO2-EG.
stretching vibration [58]. The peak at 1630 cm1 was the result of the bending vibration of water molecules adsorbed in the samples. After impregnation with nickel nitrate, all the sample (Fig. 7 curves a-d2) exhibited a strong and sharp absorption 1 band from symmetric NO 3 stretch (1360 cm ) [59]. For the EGcontaining catalysts (Fig. 7 curves b2, c2 and d2), a shoulder peak at 880 cm1 [60] was associated with the eOH band from EG. As for the CA-containing catalysts (Fig. 7 curves b1-d2), two other bands were observed at 1560 and 1720 cm1. The former was attributed to asymmetric vibration of COO, while the latter was the characteristic of the protonated COOH groups [51]. The IR bands associated to the symmetric vibrations of carboxylate and carboxylic groups around 1417 cm1 were merged with the broad absorption band from nitrate ions at 1360 cm-1. When CA was added in a higher amount (Fig. 7 curves c1, c2, d1 and d2) the main IR band for CA was at 1720 cm1, implying that the molecule remained protonated. For low CA/Ni2þ molar ratio (curve b1, b2), no protonated COOH group was observed. This IR vibration intensity increased with the CA loading, since CA contains three COOH functional groups and only two of them are needed to react with Ni2þ during the preparation of the solution [51]. For the Ni/SiO2 containing CA or EG, the SieOeSi band (1100, 800, 480 cm1) became strong (Fig. 7), indicating that the CA and EG significantly modified surface properties of silica support. The enhanced SieOeSi band of silica surface increased the negative charge on the pretreated silica surface, strengthened the interaction between supported nickel and silica, and resulted in a higher nickel dispersion [61]. These results proved that the pretreatment of silica support by EG and CA significantly improved the dispersion of the supported Ni. The catalytic activity of NiCA-x/SiO2 and NiCA-x/SiO2-EG were enhanced by the addition of CA and EG, which was
Fig. 8 e The Ni 2p XPS profiles of used NiCA-x/SiO2 and NiCA-x/SiO2-EG catalysts with different CA loadings. (A) Ni/ SiO2, (B) NiCA-0.25/SiO2, (C) NiCA-0.25/SiO2-EG, (D) NiCA1.5/SiO2, (E) NiCA-1.5/SiO2-EG.
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Fig. 9 e XRD patterns of used NiCA-x/SiO2 and NiCA-x/ SiO2-EG catalysts with different CA loadings. (a) Ni/SiO2, (b1) NiCA-0.25/SiO2, (c1) NiCA-1.0/SiO2, (d1) NiCA-1.5/SiO2, (c2) NiCA-1.0/SiO2-EG, (b2) NiCA-0.25/SiO2-EG, (d2) NiCA-1.5/ SiO2-EG.
Characteristic XRD peak for graphic carbon (2q ¼ 26.6 ) was not observed in all the spent catalysts, indicating no carbon deposition was formed during the reaction. For Ni/SiO2 catalyst, the phase corresponding to Ni was observed, suggesting that NiO was reduced to metallic Ni during the whole reaction process. The diameter of Ni particles over used Ni/SiO2 was larger than the reduced catalyst (29.7 versus 20.8 nm) (Table 1) and the particle distribution was mainly concentrated over the range of 15e30 nm (Fig. 10A), indicating that the sintering of the nickel particle was occurred and accounted for the declination in the catalytic performance. For three NiCA-x/SiO2 catalysts (Fig. 9A), the diffraction peaks assigned to NiO was observed. It could be assumed that CA favored the formation of nickel oxide under the presence of oxygen during the reaction. However, compared with the reduced catalysts, the change in NiO particle size could be negligible (7e11 versus 6.8e10.5 nm) (Table 1) and the NiO did not result in the decline of the catalytic performance. It should be noted that for the NiCA-x/SiO2-EG catalysts (Fig. 9B), along with the increase of the citric acid loading, the metal oxides in the catalysts were gradually reduced to metallic Ni during the reaction process. When x ¼ 0.25, the XRD patterns showed only NiO diffraction peak, with particle size of 10.6 nm (Table 1). With increasing CA loading to x ¼ 1.0 and 1.5, the diffraction peak of Ni appeared and NiO completely disappeared. Most importantly, the nickel particles for these two spent catalysts showed no significantly increased (Table 1). Specifically, the
Fig. 10 e TEM images of the spent Ni/SiO2 (A) and NiCA-1.5/ SiO2-EG (B) catalysts.
distribution of nickel was homogenous and located at 2e5 nm (Fig. 10B), suggesting that the sintering and oxidation of Ni species during the reaction were effectively suppressed by the co-existence of EG and higher loading of CA. This synergic effect can enhance the interaction between metal and carrier so as to prevent metal-oxide from oxidation and sintering. Since the EG and CA modification is feasible to conduct in the realworld application, the NiCA-x/SiO2-EG catalysts tested in this study exhibited a great potential to produce syngas in the oxidative CO2 reforming of methane.
Conclusion The catalytic activity of Ni/SiO2 catalysts modified with organic agents (CA and EG) for the oxidative CO2 reforming of methane was comprehensively investigated in this study. There were three major conclusions: First, the best alcoholic effect was reflected by EG which was due to the supported metal species interacting with strong SiO-Si group, leading to the formation of smaller metal particles. Second, the appropriate CA concentration in the impregnation solution was favorable for retaining the metallic nickel and preventing the aggregation of the nickel particles in the methane reforming involved with oxygen. The results demonstrated a simple way to produce small nickel particle size (about 4 nm) with high metal dispersion by modification Ni/SiO2 with CA and EG. Third, the synergic modification effect of CA and EG over NiCA-x/SiO2-EG catalysts showed higher catalytic performance and better stability than NiCA-x/SiO2 catalyst.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 8 0 8 1 e8 0 9 2
NiCA-1.5/SiO2-EG with nickel particles of 4 nm reflected the best catalytic activity and stability. [14]
Acknowledgment The authors are grateful to the financial supports from the National Natural Science Foundation of China (project No. 21173086 and U1301245) and Guangdong Natural Science Foundation (Project No. 2014A030313259).
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