Article pubs.acs.org/EF
Co-methanation of Carbon Oxides over Nickel-Based CexZr1−xO2 Catalysts Rauf Razzaq, Chunshan Li,* Nadeem Amin, Suojiang Zhang,* and Kenzi Suzuki Beijing Key Laboratory of Ionic Liquids Clean Processes, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ABSTRACT: A new NH3-reduction method was developed for the preparation of Ni−Ce0.12Zr0.88O2 catalysts with different Ni loadings (5, 10, and 15 wt %). The reduction of the catalysts was performed in a tube furnace with NH3 or H2. The catalysts were characterized using Brunauer−Emmett−Teller, inductively coupled plasma−optical emission spectroscopy, X-ray diffraction, transmission electron microscopy−high-resolution transmission electron microscopy/energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, hydrogen temperature-programmed desorption, and CO pulse chemisorption techniques. The catalytic activity toward the CO and CO2 methanation reaction was investigated. The results showed that the NH3-reduction method lead to a higher active metal reducibility, smaller Ni0 crystallite size, and higher metal dispersion compared to the H2-reduction method with 100% CO and 97% CO2 conversions and ≥98% CH4 selectivity at 250 °C.
1. INTRODUCTION As a synthetic fuel, methane has been considered a potential future energy carrier because of its high production efficiency, high calorific value, and environmental friendliness. With the abundant coal reserves around the world, coal-to-synthetic natural gas (SNG) can be regarded as a major chemical process for producing SNG.1−5 Coke oven gas (COG), a byproduct produced during coke fabrication, consisting of 55 vol % H2, 25 vol % CH4, 7 vol % CO, 3.5 vol % CO2, and a small amount of N2, can be converted to SNG by catalytic hydrogenation of carbon oxides, which is otherwise known as methanation (COx + xH2 = CH4 + xH2O).3,6,7 In fact, methanation has been widely studied and applied as a key method for large-scale gas purification in ammonia plants as well as hydrogen streams in refineries and ethylene plants. Moreover, methanation has also been recently proposed as a highly efficient way of removing even small traces of CO in H2-rich gas in fuel cell applications.8−10 The methanation of carbon oxides has been extensively investigated over a variety of transition- and noble-metalsupported catalysts.11−17 Although Ru-based catalysts are considered to be the most active for syn-gas methanation, the associated high costs of Ru catalysts limit their large-scale commercial use. Recently, Ni is the most widely applied and studied methanation catalyst because of its high activity, selectivity, and relatively low cost. CeO2−ZrO2 composite oxides are considered to be effective support materials because of their high thermal stability, oxygen storage capacity, and effectiveness in suppressing coke formation.18,19 Ni-based catalysts supported by CexZr1−xO2 binary oxides prepared through the sol−gel method have been previously investigated for CO2 methanation.20−23 High NiO dispersion, Ni2+ incorporation into the mixed oxide structure, and formation of additional active centers on the surface of the catalyst lead to an increased CO2 methanation activity. Although the methanation of carbon oxides has been extensively investigated, studies on the co-methanation of CO © 2013 American Chemical Society
and CO2 are limited. Moreover, the effect of the type of reducing gas on methanation activity has not been carried out before. In this study, we report the methanation of CO and CO2 over nickel-based CexZr1−xO2 catalysts prepared via the co-precipitation method. The effect of the type of reducing gas (NH3 and H2) used for the reduction of the catalyst precursor on the methanation activity of carbon oxides was investigated. The activity was tested in terms of the percentage CO and CO2 conversion and CH4 selectivity.
2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. All gases and reagent chemicals were commercially obtained and used without further purification. Distilled water was used to prepare all aqueous solutions. Ni−Ce0.12Zr0.88O2 catalysts were synthesized using the conventional co-precipitation method. Nickel nitrate, Ni(NO3)2·6H2O (Santou Xilong Chemicals, Ltd., ≥98.0%), was used as an active metal precursor. Cerium nitrate, Ce(NO3)3·6H2O (Sinopharm Chemical Reagent Co., Ltd., 99.0%), and zirconium oxychloride, ZrOCl2·8H2O (Sinopharm Chemical Reagent Co., Ltd., 99.0%), were used as sources of Ce and Zr, respectively. Ni loading was varied from 5 to 15 wt %. All components were precipitated from the solution using 100 mL of aqueous solution of 0.2 M KOH with constant stirring for 2 h. The pH of the solution was maintained at around 10, and the samples were aged overnight. The samples were then centrifuged and washed with distilled water to remove extra water and other unwanted ions. The precipitate was dried in an oven at 120 °C for 8 h. The resulting solid was calcined in air at 600 °C for 4 h. All calcined samples were then loaded in a tube furnace with approximately 2 g of the oxide precursor, while NH3 (90 mL/min, 99.99%) or H2 (90 mL/min, 99.99%) was passed over the samples in a temperature-programmed reaction. The reaction was carried out at 400 °C for 3 h. The furnace was then cooled and passivated in 1% O2/N2 for 9 h. The resulting NH3-treated catalytic systems were named 5Ni/CZ(A), 10Ni/CZ(A), and 15Ni/ CZ(A), while H2-treated samples reduced at 400 °C were named 5Ni/ Received: June 4, 2013 Revised: October 10, 2013 Published: October 14, 2013 6955
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Table 1. Metal Content, Surface Area, Average Pore Diameter, Total Pore Volume, and Ni0 Crystal Size for NH3- and H2Treated Ni/CZ Catalysts Ni content (wt %)
a
catalyst
EDX
ICP−OES
surface area (m2 g−1)a
average pore diameter (nm)a
total pore volume (cm3 g−1)a
CZ 5Ni/CZ(A) 10Ni/CZ(A) 15Ni/CZ(A) 5Ni/CZ(H) 10Ni/CZ(H) 15Ni/CZ(H) 15Ni/CZ(H, 500 °C)
5.9 10.6 15.8 6.1 11.2 16.1 15.9
4.7 9.1 13.7 4.3 8.9 13.4 13.6
35 32 38 52 29 32 43 41
9.6 12.0 9.5 7.8 12.2 9.5 9.6 9.4
0.09 0.10 0.09 0.11 0.11 0.09 0.11 0.10
Ni0 diameter (nm)b
7.1 13.6
19.4 20.6
Determined by N2 adsorption/desorption measurement. bCalculated from the Ni0 (111) plane using the Scherrer equation from XRD. CO, CO2, and CH4) were analyzed at temperatures between 200 and 400 °C. The percentage conversion of CO and CO2 were calculated using eqs 1 and 2 to determine the catalytic activity. The product selectivity (SCH4) was determined using eq 3
CZ(H), 10Ni/CZ(H), and 15Ni/CZ(H), and the H2-treated sample reduced at 500 °C was named 15Ni/CZ(H, 500 °C). 2.2. Catalyst Characterization. The specific surface areas (SSAs) of the catalysts were determined from the adsorption−desorption profile of N2 at the liquid N2 temperature using a Quadrasorb SI-MP equipment (Quantachrome, U.K.). X-ray diffraction (XRD) was performed on a Bruker D8 Focus X-ray diffractometer with Ni-filtered Cu Kα (λ = 0.1541 nm) radiation. The scanned 2θ ranged between 10° and 80° with 0.015 increments and 1 s measuring time at each increment. The nickel content in the samples was determined using inductively coupled plasma−optical emission spectroscopy (ICP−OES, Shimadzu’s ICPE-9000). The samples were digested using sulfuric acid diluted with deionized water. Transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDX) was performed using JEM-2010 at an acceleration voltage of 200 kV. A high-resolution transmission electron microscopy (HRTEM) image was obtained at a higher magnification with an acceleration voltage of 200 kV. The samples were prepared via ultrasonic dispersion in ethanol with a drop of the resultant suspension evaporated onto a holey carbon-supported grid. Scanning electron microscopy (SEM) was performed using a FEIMLA250 system with an acceleration voltage of 20 kV at a high-vacuum operating mode. X-ray photoelectron spectroscopy (XPS) data were obtained using an ESCALab220i-XL electron spectrometer (VG Scientific) with 300 W Al Kα radiation. The base pressure was approximately 3 × 10−9 mbar. The binding energies (BEs) were referenced to the C 1s line at 284.8 eV from the adventitious carbon. Hydrogen temperature-programmed desorption (H2-TPD) was performed on a Micromeritics Autochem II 2920 unit equipped with a thermal conductivity detector (TCD). Before H2-TPD experiment, 100 mg of the catalyst was pre-reduced at 400 °C at a heating rate of 2 °C min−1 and held for 60 min under 10% H2/Ar flow and another 90 min in pure Ar to minimize H2 spillover. The sample was then cooled to room temperature, and a H2-TPD experiment was performed at 800 °C with a heating rate of 15 °C min−1 under pure Ar flow. The amount of chemisorbed CO was determined through a pulse method using Micromeritics Autochem II 2920 unit equipment. The catalyst (100 mg) was first reduced in situ at 300 °C for 1 h under 10% H2/Ar flow. The sample was then cooled to room temperature under Ar flow. Pulses of 1% CO in He gas were injected through the catalysts at room temperature until no further adsorption of CO was detected using a thermal conductivity detector (TCD). 2.3. Activity Test. All experiments were performed in a fixed-bed quartz tubular reactor (10 mm inner diameter) at 2.0 MPa pressure. About 0.7 g of the sample (30−40 mesh) was introduced into the reactor tubing. A gas mixture containing 7.04% CO, 3.05% CO2, 4.05% N2, 27.23% CH4, and 58.63% H2 (similar to the composition of COG) was continuously passed through the catalysts with a gas hourly space velocity of 5000 h−1. After the reaction, the gas mixture was analyzed using an online gas chromatograph equipped with a TCD. The produced gases (H2, N2,
CO conversion
⎛ [M ] − [MCO]out ⎞ XCO (%) = ⎜ CO in ⎟ × 100 [MCO]in ⎝ ⎠
(1)
CO2 conversion
⎛ [MCO ]in − [MCO ]out ⎞ 2 2 ⎟⎟ × 100 XCO2 (%) = ⎜⎜ [ ] M ⎝ ⎠ CO2 in
(2)
CH4 selectivity
⎛ ⎞ [MCH4]out − [MCH4]in ⎟⎟ × 100 SCH4 (%) = ⎜⎜ ⎝ [MCO + CO2]in − [MCO + CO2]out ⎠
(3)
where M is the molar concentration of the inlet and outlet species and S is the CH4 selectivity with respect to CO and CO2. Thermodynamic equilibrium calculations were also performed at various temperatures ranging from 200 to 400 °C to compare our catalytic results to thermodynamic limitations.
3. RESULTS AND DISCUSSION 3.1. Surface Area and Textural Properties. Several textural and structural properties of different catalysts are shown in Table 1. For all samples, the actual metal loading was tested using ICP−OES and EDX analysis. From ICP, it was revealed that the Ni content was slightly lower than the designed value. However, our EDX analysis showed a higher metal content, indicating that Ni was more abundant on the surface of the catalyst rather than inside. All catalysts had a type IV isotherm (not shown), which is typically associated with mesoporous solids. The Ce0.12Zr0.88O2 support had a specific surface area of about 35 m2 g−1, which increased upon higher metal loading. The pore volume first decreased when the Ni loading was increased from 5 to 10 wt %, followed by an increase at higher (15 wt %) metal content. The increment of the Brunauer−Emmett−Teller (BET) surface area for all prepared catalysts indicates no physical blockage and/or pore collapsing when the Ni loading was increased from 5 to 15 wt %. At higher metal loading, some extra Ni nanoparticles that are not inserted into the pores are probably located on the outer surface of the oxide support. These Ni nanocrystals may result in a higher SSA of the catalyst.22 6956
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3.2. Catalyst Reducibility [Hydrogen TemperatureProgrammed Reduction (H2-TPR)]. H2-TPR was performed for the as-calcined samples to study catalyst reducibility. Results are shown in Figure 1. The total H2 consumption was 11.86,
Figure 2. XRD spectra for NH3- and H2-reduced samples.
completely in H2 at 400 °C. Therefore, the as-calcined 15Ni/ CZ sample was reduced under H2 flow at a higher reduction temperature (500 °C) to obtain complete metal reduction. Table 1 shows the average crystal diameters of Ni0 that were estimated from the (111) plane using the Scherrer equation. The Ni0 crystallite size for the 15Ni/CZ(A) sample was significantly lower compared to 15Ni/CZ(H) and 15Ni/CZ(H, 500 °C) reduced at 400 and 500 °C, respectively. Moreover, the crystal size also increased with increasing metal loading. All samples exhibited the presence of CexZr1−xO2 as a tetragonal phase. 3.4. TEM and EDX Analysis. The TEM images of 15Ni/ CZ(A) and 15Ni/CZ(H) are shown in panels a and b of Figure 3, respectively. The catalysts consisted of nanosize particles with nearly spherical geometry. The 15Ni/CZ(A) sample exhibited an average particle size of 16 nm, whereas agglomeration occurred in the 15Ni/CZ(H) sample. Thus, the latter had a higher particle size (22 nm). The results are in good agreement with the XRD crystalline sizes shown in Table 1. However, from TEM images, it was difficult to identify Ni nanoparticles because they were probably well-dispersed within the CexZr1−xO2 matrix. HRTEM analysis (Figure 3c) was performed to distinguish the Ni nanoparticles and CexZr1−xO2 solid solution. The HRTEM image of 15Ni/CZ(A) clearly identifies two lattice fringes, with the lattice spacing measured as ca. 0.29 and 0.20 nm, which are in agreement with the d spacing of Ce0.12Zr0.88O2 (101) and Ni (111) planes, respectively. Further, the EDX analysis of the same area also confirmed the existence of Ni, Ce, and Zr components. 3.5. SEM Analysis. The surface morphology for NH3- and H2-reduced samples was studied using SEM (not shown). The SEM images revealed that the H2-reduced sample exhibits a larger grain size compared to the NH3-reduced sample, possibly because of aggregation of smaller particles and/or metal sintering, as confirmed by XRD and TEM analyses. 3.6. XPS Analysis. The NH3- and H2-reduced 15Ni/CZ samples were also studied by XPS. During the nitridation treatment (400 °C) using NH3, the metastable nickel nitrides decompose to generate metallic Ni in the bulk phase while releasing nitrogen under the following reaction:26
Figure 1. H2-TPR profile of CZ, 5Ni/CZ, 10Ni/CZ, and 15Ni/CZ samples.
23.17, 36.85, and 49.80 cm3 g−1 of catalyst for CZ, 5Ni/CZ, 10Ni/CZ, and 15Ni/CZ, respectively, indicating an increase in metal reducibility at a higher Ni content. The TPR profile of the CZ support shows a weak broad peak at 560 °C, which can be attributed to the reduction of CeO2 (from Ce4+ to Ce3+).20,21 For 5Ni/CZ and10Ni/CZ samples, two reduction peaks were observed, while the 15Ni/CZ sample exhibited a single reduction zone. Low-temperature peaks are assigned to the reduction of weakly interacting and/or free NiO species, whereas higher reduction peaks are attributed to the reduction of complex NiO species having a strong interaction with the support material.24,25 The 5Ni/CZ sample shows two reduction peaks centered at 204 and 347 °C, indicating the presence of both weakly and strongly interacting Ni species. As the metal loading was increased, both reduction peaks shifted toward a higher temperature, indicating a strong metal support interaction for the 10Ni/CZ sample. The peak centered at 370 °C is attributed to the reduction of surface NiO, whereas the peak centered at 458 °C can be attributed to the reduction of Ni in a strong interaction with the mixed oxide support. The 15Ni/CZ sample exhibited a single reduction peak at 365 °C, indicating a weak metal support interaction and resulting in higher metal reducibility. 3.3. Crystalline Phases (XRD). The XRD patterns of the catalysts reduced by NH3 and H2 are shown in Figure 2. The samples reduced by NH3 had diffraction peaks at 2θ = 44.508°, 51.845°, and 76.372°, which are assigned to metallic Ni [Joint Committee on Powder Diffraction Standards (JCPDS) card number 04-0850], indicating the reduction of NiO species to metallic Ni. As expected, the diffraction peaks of metallic Ni gradually became more intense and sharper when the Ni loading was increased from 5 to 15 wt %. The H2-reduced samples exhibited low intensity reflections at 37.312°, 43.290°, and 62.926°, which are attributed to NiO. However, the diffraction pattern of the 15Ni/CZ(H) sample also showed a distinct peak at 2θ = 44.508°, indicating the reduction of NiO particles. XRD patterns revealed that NiO was not reduced
2Ni3N = 6Ni + N2 Figure 4 shown N 1s XPS spectra for 15Ni/CZ(A) and 15Ni/ CZ(H) samples. The H2-reduced sample was unable to detect 6957
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Figure 4. N 1s XPS spectra for 15Ni/CZ(A) and 15Ni/CZ(H) samples.
indicating a higher concentration of N2 on the surface of the catalyst during NH3 reduction. 3.7. H2-TPD. Figure 5 shows H2-TPD curves for NH3- and H2-reduced samples. Both samples exhibit three desorption
Figure 5. H2-TPD profiles of 15Ni/CZ(A) and 15Ni/CZ(H) catalysts.
peaks, indicating more than one active site for H2 adsorption. The low-temperature desorption peak (
