Promoter Effects on Nickel-Supported Magnesium Oxide Catalysts for

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Promoter Effects on Nickel-Supported Magnesium Oxide Catalysts for the Carbon Dioxide Reforming of Methane Dedong He, Yongming Luo, Yongwen Tao, Vladimir Strezov, Peter F Nelson, and Yijiao Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02361 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Promoter Effects on Nickel-Supported Magnesium Oxide Catalysts for the Carbon Dioxide Reforming of Methane Dedong He a,b, Yongming Luo c, Yongwen Tao d, Vladimir Strezovb,e, Peter Nelsonb,e, Yijiao Jianga,b.*

a

Department of Engineering, Macquarie University, Sydney, NSW 2109, Australia

b

Energy and Environmental Contaminants Research Center, Sydney, NSW 2109, Australia

c

School of Environmental Engineering, Kunming University of Science and Technology,

Kunming, 650093, China d

School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney,

NSW 2006, Australia e

Department of Environmental Science, Macquarie University, Sydney, NSW 2109, Australia

*To whom correspondence should be addressed: Tel: +612-9850-9535 Fax: +612-9850-9128 E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract The nickel catalysts supported on the bare MgO and its binary Mg-Al, Mg-La, and Mg-Fe metal oxides were prepared and used for carbon dioxide reforming of methane to syngas. The effects of Al, La, and Fe metal oxides on the structural properties, reducibility and metal-support interaction of the Ni catalysts supported on the MgO-based binary metal oxide were investigated. The XRD, TEM, and H2-TPD analyses show that the nickel nanoparticles were highly dispersed on the supports. It is found that the Al ions can be well incorporated into the MgO lattice to form uniform Mg-Al oxides, while isolated lanthanum oxides and iron oxides were observed in the Mg-La and Mg-Fe binary systems by TEM, respectively. The Ni/Mg-Al metal oxide exhibits greatly improved catalytic activity owing to the formation of homogeneous Mg-Al oxide matrix with small particle sizes of Ni nanoparticles compared to the bare Ni/MgO. Very low conversions for both CH4 and CO2 were obtained on the Ni/Mg-La and Ni/Mg-Fe metal oxides even at a high temperature of 800 ℃ due to the incomplete reduction of the nickel nanoparticles.

Keywords: CO2 reforming of methane; Nickel catalyst; Magnesium oxide; Binary metal oxide; Al promoter effect.


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1. Introduction Carbon dioxide (CO2) and methane (CH4) have been identified as the two primary greenhouse gases that cause the increase in global temperature and climate disruption. In recent years, there has been great interests in the reduction and utilization of these two greenhouse gases.1 The catalytic process of CO2 reforming of CH4 (CRM) into syngas, a mixture of hydrogen and carbon monoxide provides one of the prospective routes for utilizing CH4 and CO2 sources. The produced syngas in this process (CH4 + CO2 → 2H2 + 2CO) has a low H2/CO ratio close to 1, which is suitable for many important industrial processes, such as Fischer-Tropsch synthesis and varied hydrocarbons production.2 Supported noble metal catalysts and nickel-based catalysts have been extensively studied for the CRM process.3 Among them, the nickel-based catalysts are one of the most promising candidates for the CRM reaction due to their high activity, low cost, and abundance.4,5 Basic oxides such as lanthanum oxide (La2O3), zirconium oxide (ZrO2), and magnesium oxide (MgO) have been widely investigated as supports for nickel oxide catalysts in the drying reforming process.6-9 More CO2 are expected to be adsorbed and activated on these supports attributed to their basic nature.10 MgO is considered as a promising basic oxide support and has recently been attracting renewed interests since it was proven to be stable and actively participate in the CRM reaction.11 Both MgO and nickel oxide (NiO) are faced-centered oxides with cubic type. The bond distances and lattice parameters of MgO are close to that of NiO.12,13 The strong metal-support interaction between MgO and NiO may hinder the reduction of NiO to Ni, thus a non-reducible Ni-Mg oxide is supposed to be formed.14 Consequently, the Ni particles supported on the magnesium oxide are often small


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and the Ni particle sizes can be controlled accordingly. Furthermore, stable and highly-dispersed active Ni sites can probably be achieved because of the strong metal-support interaction.14 However, too strong interaction between metal and support might limit the accessibility of the Ni active sites during the reaction. To achieve high catalytic performance, modulation of the interaction between Ni nanoparticles and MgO-based support is desirable. It is well accepted that support compositions can affect the reducibility, textural properties, and the metal-support interaction of nickel-based catalysts. Binary metal oxides have exhibited better performance compared to their single counterparts.15 Adding La2O3 into MgO can lead to the formation of strong basic sites on the surface of binary Mg-La oxides, resulting in an increased basicity of the supports.16,17 The presence of La2O3 will promote the chemisorption and dissociation of CO2 on the Ni catalyst surface.14 On the other hand, the presence of acidic sites on the catalyst is supposed to favor the dissociation of the adsorbed CH4 and CO2 species during the CRM process.5 Alumina (Al2O3) is considered as a promising acidic support for Ni catalysts since they can provide acidic sites for the CRM reaction.18,19 The addition of Al2O3 into MgO to form Mg-Al binary metal oxide catalysts can effectively promote the dispersion of metals on the supports, resulting in a good activity of the catalysts.20 Additionally, ferrite oxide (FeO) has the comparable lattice parameters and similar crystal structure as MgO,13 and the Mg-Fe binary metal oxides has been found to be efficient for several catalytic reactions.21 The investigation of Mg-Fe binary metal oxides as the support of Ni catalysts for the CRM reaction has not yet been reported. In the present work, the nickel catalysts supported on the bare MgO and its binary Mg-Al, Mg-La, and Mg-Fe metal oxides were prepared and used for the CO2 reforming of


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CH4 to syngas. The main focus of this work is to investigate the effects of Al, La, and Fe metal oxides on the structural properties, the reducibility, and the metal-support interaction of the Ni catalysts supported on the MgO-based binary metal oxide.

2. Experimental 2.1. Catalyst preparation The binary Mg-Al, Mg-La, and Mg-Fe metal oxides were prepared by co-precipitation of magnesium nitrate and Me (Me = Al, La, or Fe) nitrates with a molar ratio Mg/Me (Me = Al, La, or Fe) of 4. A typical synthetic procedure is as follows: nominal amounts of magnesium nitrate and Me (Me = Al, La, or Fe) nitrates with a concentration of 1.5 M were dissolved in the Milli-Q water. The mixture solution was added drop wise into the Na2CO3 solution (1 M) under vigorous stirring. The pH value was adjusted to 10 by adding the NaOH solution (10 M). The obtained mixture was kept in an oven at 75 oC for 18 h. Subsequently, the resulting gel was filtered and washed several times with the Milli-Q water, and then dried at 120 oC for 10 h. Finally, the samples were calcined at 460 oC for 5 h to obtain the corresponding metal oxides. For comparison, the bare MgO support was also prepared by using the same procedure. 5 wt% of nickel nitrate was loaded on the above-prepared catalysts by a wet impregnation method. The mixture was dried at 120 oC for 10 h and subsequently calcined under an air flow at 850 oC for 5 h. The obtained catalysts were denoted as Ni/MgO, Ni/Mg-Al, Ni/Mg-La, and Ni/Mg-Fe, respectively. 2.2. Catalyst characterization


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X-ray powder diffraction (XRD) patterns were recorded on a Philips X’pert multipurpose diffractometer using monochromatic Cu Kα radiation (λ=0.154 nm), operated at 45 kV and 40 mA. The diffraction angle (2θ) was ranged from 20 to 80 o. High-resolution transmission electron microscopy (HR-TEM) was performed on a Philips CM 200 facility operating at 200 kV. The high-angle annular dark field scanning TEM (HAADF-STEM) images and energy dispersive X-ray spectra (EDX) were recorded on a JEOL JEM-ARM200F TEM with an EDX detector. Hydrogen temperature-programmed reduction (H2-TPR) was conducted on a thermo-gravimetric analyzer instrument. All the samples (10 mg) were pre-treated under a nitrogen flow at 400 ℃ for 1 h prior to the TPR analysis. A 5 % H2/N2 flow (20 ml/min) was used as the reducing gas. The temperature was increased from 100 to 1000 ℃ at a ramp rate of 10 ℃/min. X-ray photoelectron spectroscopy (XPS) of the catalysts was performed on an ESCALAB250Xi spectrometer (Thermo Scientific, U.K.) using a monochromated Al Kα X-ray radiation source at 15.2 kV and 168 W. The binding energies were calibrated on the basis of the hydrocarbon C1s peak at 285.0 eV. The spectra deconvolution was carried out by XPS PEAK41 software packages. 2.3. Catalytic activity test The catalytic CO2 reforming of CH4 was carried out under atmospheric pressure in a fixed-bed stainless steel reactor (300 mm long and 6 mm ID) packed with 100 mg of catalyst. The catalyst was pre-heated under a nitrogen flow (20 mL/min) to 750 °C (heating rate 10 °C/min) and held at this temperature for 1 h. Prior to the reaction, the catalyst was in situ


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reduced at 750 ℃ for 1 h with a flow rate of 50 % H2/N2 mixture gas (10 mL/min N2 and 10 mL/min H2) and cooled down to 600 ℃. Then the reactant gases consisted of CH4 (15 mL/min), CO2 (15 mL/min) and N2 (15 mL/min) with a total flow of 45 mL/min were introduced into the reactor. The reaction temperature was increased from 600 to 800 ℃ in 50 ℃ increments with a heating rate of 10 ℃/min and held for 0.5 h until the conversion was stabilized at each temperature. The stability test of the samples was conducted at 700 ℃ for 10 h. A Varian 490 Quad Micro-GC equipped with three thermal conductivity detectors (TCD) with three columns (Agilent PoroBond Q, CP-Molsiever 5A, and HayeSep Q) was used for H2, CO, CO2, and CH4 analysis.

3. Results and discussion 3.1 XRD results Figure 1 shows the XRD patterns of the Ni catalysts supported on the bare MgO, and the binary Ni/Mg-Al, Mg-La, Mg-Fe metal oxides. For the Ni/MgO catalyst, clear reflections at about 2θ = 36.5, 43.0, 62.3, 74.7 and 78.6o represent MgO crystal planes of (111), (200), (220), (311), and (222) (JCPDS card no. 87-0653). This crystal structure is attributed to the single phase face-centered cubic MgO.22 It cannot exclude the presence of NiO particles due to their similar diffraction peaks occurring at 2θ = 37.6, 43.7, 63.2, 75.4 and 79.4o corresponding to NiO (111), (200), (220), (311), (222) according to the reported data (JCPDS card no. 47-1049). Keeping the same content of NiO particles (5 %) on the various supports of Mg-Al, Mg-La, Mg-Fe metal oxides, the peak intensities were inconsistent. The XRD patterns of both Ni/Mg-La and Ni/Mg-Fe samples exhibit appreciable


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differences with respect to the Ni/MgO sample. Ni/Mg-La shows characteristic reflections of La2O3 and MgLa2Ox in addition to MgO phases.17 The MgFe2Ox spinel phase are observed for the Ni/Mg-Fe sample.21 The XRD pattern of Ni/Mg-Al sample is quite similar to that of Ni/MgO with characteristic reflections at 2θ = 37.1, 43.1, 62.6, 74.9, and 78.9o, indicating the formation of Mg-Al metal oxides with face-centered cubic structure.24 No diffraction peaks assigned to Al2O3 or MgAl2Ox spinel are observed, which indicates that Al ions might be well incorporated into MgO lattice.24,25 The incorporation of Al3+ into MgO lattice is attributed to the smaller ionic radii of Al3+, i.e. 0.50 Å compared to Fe3+ (0.69 Å) and La3+ (1.15 Å). The ionic radius of Al3+ is smaller than that of Mg2+ (0.65 Å). Therefore, Al3+ ions can partially substitute the lattice of Mg2+ cations without segregation of Al3+ species.

Figure 1. XRD patterns of Ni catalysts supported on the bare MgO, and the binary Mg-La, Mg-Fe and Mg-Al metal oxides


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3.2 TEM measurements Figure 2a shows the TEM image of the Ni/Mg-Al catalyst. No typical cubic type structure of MgO is found, indicating that the homogeneous Mg-Al mixed oxides with small particle sizes are formed, which corroborates well with the XRD result. EDX mapping of Mg and Al of the Ni/Mg-Al sample is provided in Figure S1 (a) and (b), to further confirm the uniform distribution of Mg and Al elements (Supporting Information). The Ni mapping in Figure 2b displays the highly dispersed Ni nanoparticles. The HR-TEM image in Figure 2c shows that the well-crystalized Ni nanoparticle with a size of ca. 15 nm is dispersed on the surface of Mg-Al oxides. As shown in Fig. S2, the insert image shows that the lattice spacing is ca. 0.201 nm, close to the (111) interplanar distance of nickel, confirming the presence of nickel particle.

Figure 2d exhibits the TEM image of the Ni/Mg-La catalyst. The typical cubic type structure of MgO is apparently observed on this sample. In Figure 2e, the presence of a dark area around the cubic structure of MgO suggests that the isolated La species were formed, corroborating well with the XRD result. The EDX mapping of La confirms that the dark region is the isolated La species (La2O3, with cubic crystal structure) as shown in Figure 2f. Also, the EDX mapping of Mg and La of the Ni/Mg-La catalyst in Figure S1 (c) and (d) indicates that La element does not show the uniform distribution as the Al element (Supporting Information), which may be attributed to the formation of the isolated La species.


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Figure 2 (a) TEM image of Ni/Mg-Al metal oxide, (b) elemental mapping of Ni, (c) HR-TEM of Ni nanoparticle on the Mg-Al support, (d) TEM image of Ni/Mg-La metal oxide, (e) TEM image of La2O3 particle, and (f) elemental mapping of La

3.3 H2-TPR measurements The H2-TPR profiles of Ni/MgO, Ni/Mg-La, Ni/Mg-Fe and Ni/Mg-Al metal oxides are presented in Figure 3. The Ni/MgO catalyst (Fig. 3a) shows a profile with three reduction peaks. The lower temperature peak at 250-350 oC corresponds to the reduction of surface Ni3+ species, which are usually represented for the reactive surface oxygen species on the catalyst.22,26 Besides, the peaks at higher temperatures of about 520 oC can be attributed to the reduction of the outermost and sub-layer Ni2+ species, respectively. For the Ni/Mg-La catalyst (Fig. 3b), the first peak at 350 oC is associated with partial hydrogen consumption from the


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reduction of Ni3+ in the perovskite-type structure (as previously observed for LaNiO3 phase from the XRD pattern) to Ni2+.27 The peak observed at temperatures of 500 oC is assigned to the further reduction of LaNiO3 from Ni2+ to Ni0, while the peak at 590 oC is due to the reduction of Ni oxide species that strongly interact with the structural defects of lanthanum oxides. In addition, the high-temperature peak at above 800 ℃ might be attributed to the reduction of Ni2+ ions located in the MgO lattice, in good agreement with the literature.26 In Fig. 3c, the H2-TPR profile of Ni/Mg-Fe sample exhibits broad reduction peaks between 500-600 ℃, which may be also attributed to the reduction of Ni2+ species in the outermost layer and sub-surface layers of the Mg-based support lattice. For Ni/Mg-Al metal oxides (Fig. 4d), a very broad peak at temperatures between 200-500 ℃ is observed. It seems that the reduction process of the outermost and sub-layer Ni2+ species cannot be distinguished due to the strong interaction with the Mg-Al metal oxide support.22 It is noted that a reduction peak occurred at 200 ℃, implying that more reactive surface oxygen species exist on the Ni/Mg-Al oxides.


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Figure 3 H2-TPR profiles of the Ni/MgO (a), Ni/Mg-La (b), Ni/Mg-Fe (c), and Ni/Mg-Al (d).

3.4 XPS analysis XPS measurements were performed for the reduced Ni/MgO, Ni/Mg-La, Ni/Mg-Fe and Ni/Mg-Al samples as shown in Figure 4. Fig. 4a displays the corresponding Mg 2p spectra for the reduced catalysts. A shift towards lower binding energy is observed after incorporating of other metals (La, Fe and Al), which reflects the change of intra-layer electron density. Deconvolution of the O1s spectra (Fig. 4b) shows two peaks corresponding to the lattice O2ions at 531-532 eV and surface oxygen species at 533-534 eV, respectively.17 The intensity of the peak assigned to the surface oxygen species is high for the Ni/Mg-La sample, while low for the Ni/Mg-Fe and Ni/Mg-Al catalysts, indicating the increased basicity of the


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La-modified catalyst. Moreover, these two peaks of the O 1s signal are not clearly separated in the spectra of the Ni/Mg-Al catalyst. Regarding the result of H2-TPR, a very broad reduction peak indicates the reduction process of the different Ni species cannot be distinguished. Hence, the result of O 1s spectra is consistent with the result of H2-TPR.

Figure 4 (a) Mg 2p and (b) O 1s XPD spectra of Ni/MgO, Ni/Mg-La, Ni/Mg-Fe, and Ni/Mg-Al samples.

Figure 5 exhibits the core level spectra of Ni 2p, La 3d, Fe 2p and Al 2p for different samples. It is noted that the Ni 2p peak is overlapped with Fe 2p and La 3d peaks. Hence, the Ni spectra were merely recorded for the Ni/MgO and Ni/Mg-Al samples. From Fig. 5a, two peaks are observed corresponding to Ni2+ and Ni3+, respectively.29 It seems that there is no significant difference of the Ni XPS profile between these two samples. For the La 3d core level spectra in Fig. 5b, the main peaks of La 3d5/2 (834 eV) and La 3d3/2 (851 eV) as well as the satellite peaks of La 3d5/2 (838 eV) and La 3d3/2 (855 eV) can be seen.30 As shown in Fig.


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5c, the peaks centered at 725 eV and 711 eV are ascribed to the Fe 2p1/2 and Fe 2p3/2, respectively, indicating the existence of Fe3+ ions in the sample.31 The Al 2p peak in Fig. 5d with the binding energy at around 75 eV indicates that Al existed mainly in the form of Al-O in the catalyst.32 Moreover, the Al 2p peak at the observed binding energy is also associated with aluminum oxide formation, which indicates that the oxidation state of Al is Al3+.33

Figure 5 XPS spectra of Ni 2p for the Ni/Mg-Al metal oxide and Ni/MgO (a), La 3d for the Ni/Mg-La metal oxides (b), Fe 2p for the Ni/Mg-Fe metal oxides (c), and Al 2p for the


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Ni/Mg-Al metal oxides (d)

3.5 Catalytic performance The catalytic activity of Ni/MgO, Ni/Mg-La, Ni/Mg-Fe, and Ni/Mg-Al metal oxides were evaluated for the CO2 reforming of methane. As shown in Fig. 6a and b, the conversions of CH4 and CO2 over Ni/MgO increased significantly with the increase of the reaction temperatures, reflecting the intensely endothermic feature of the CRM reaction. Moreover, the conversion of CO2 was found to be higher than that of CH4, indicating the simultaneous occurrence of water gas shift reaction (CO2 + H2 → H2O + CO) during the catalytic CO2 reforming of CH4. It is apparent that the Ni/Mg-Al metal oxide exhibits significantly improved catalytic activity compared to the Ni/MgO. The conversions of CH4 and CO2 over Ni/Mg-Al catalyst achieved over 95 % and 93 % at 800 ℃, respectively. More reactive surface oxygen species located on the Ni/Mg-Al oxides as evidenced by H2-TPD may contribute to the increased catalytic activity in the CRM reaction. The conversions of CH4 and CO2 over Ni/Mg-La and Ni/Mg-Fe samples were much lower than that of Ni/Mg-Al catalyst. Very low conversions for both CH4 and CO2 were presented on the Ni/Mg-La and Ni/Mg-Fe metal oxides even at a high temperature of 800 ℃. Previous report revealed that the reduction of perovskite structure with formation of Ni-La or Ni-Fe alloy and corresponding oxides may lead to more difficult reduction of the catalysts.27,28 Consequently, higher reduction temperatures may be needed for the complete reduction of the catalyst. The Ni/Mg-La sample was thus reduced in situ in a flow of 50 % H2/N2 mixture gas at 800 oC for 5 h. However, the conversions of both CH4 and


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CO2 remained low (not shown). Hence, incomplete reduction of the Ni/Mg-La and Ni/Mg-Fe catalysts under the current reaction conditions could be ascribed to the observed rather low catalytic activity accounting for low accessibility of the active Ni sites to the reactants of CH4 and CO2. As shown in Figure 6c, a long-term stability test of the Ni/Mg-Al catalyst at 700 ℃ exhibits stable and high catalytic activities, ca. 80 % for the conversions of both CH4 and CO2, respectively. It is also noted in Fig. 6d that the H2/CO ratio over Ni/MgO catalyst was lower than the stoichiometric value (1/1) of the CRM reaction. It might be due to the fact that the produced H2 was partially consumed by the water gas shift reaction during the CRM reaction.


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Figure 6 (a) CH4 conversion, (b) CO2 conversion with temperature over Ni/MgO, Ni/Mg-La, Ni/Mg-Fe, and Ni/Mg-Al metal oxides; (c) long-term stability test over the Ni/Mg-Al metal oxides at 700℃ and (d) H2/CO ratio obtained from the long-term stability test, with a feed ratio of 1:1:1, CH4: CO2: N2 and GHSV 13,500 mL/(h·gcat).

4. Conclusion In this work, the nickel catalysts supported on the bare MgO and its binary Mg-Al, Mg-La, and Mg-Fe metal oxides were prepared and used for carbon dioxide reforming of methane to syngas. The promoter effects of Al, La, and Fe metal oxides on the structural properties, reducibility and metal-support interaction of the Ni catalysts supported on the MgO-based binary metal oxide were investigated. It is found that various promoters of the Ni-Mg based catalysts gave rise to different catalytic performance. The addition of La or Fe oxide into Ni/MgO catalyst largely decreased the catalytic activity due to the formation of isolated La2O3 and incomplete reduction of nickel oxide, while Al promoter facilitated the greatly improved catalytic activity for CRM reaction thanks to the formation of homogeneous Mg-Al metal oxides with small nickel nanoparticle.

Acknowledgements Financial support through ARC Discovery Project (DP140102432) and Macquarie University Research Development Grant are gratefully acknowledged. DH is grateful for the financial support from the China Scholarship Council.

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Energy & Fuels

Graphical Abstract


Promoter Effects on Nickel-Supported Magnesium Oxide Catalysts for

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