A comparative study of dry reforming of methane over nickel catalysts supported on perovskite-type LaAlO3 and commercial α-Al2O3
Graphical abstract
Introduction
The high concentrations of CO2 in the atmosphere resulting from increasing energy consumption and mainly from burning fossil fuels have generated initiatives for the production of renewable energy and clean technologies for CO2 capture and conversion [1], [2], [3], [4]. Dry reforming of methane (DRM) stands out as a technology for the conversion of CH4 and CO2 using fuels as the natural gas or renewable gas (biogas) for the production of H2 and CO, which are the basis of the chemical and energy industries [5], [6]. Hydrogen is considered the main energy source of the future and its production has attracted attention in recent years due to the new technologies and materials for its safe storage [7], [8], [9], [10], as well as its use in fuel cells [11], [12], [13].
The DRM reaction catalyzed by metal supports (Eq. (1)) is endothermic and spontaneous starting from 640 °C [14]. The noble metals (Ru, Rh, Pd, Ir and Pt) exhibit the highest catalytic activities and stabilities for this reaction [15], but their high cost make them unfeasible for large-scale use. Different metals have been proposed as substitutes for noble metals, with special attention focusing on Ni, Fe and Co [5], [16], [17]. Nickel stands out for its good performance and relatively low cost compared to the noble metals [18], [19], but studies indicate that its catalytic stability is still limited due to sintering and the carbon deposits that cover the active sites [20]. Thus, several researches have focused on the development of nickel catalysts and on the optimization of process conditions that eliminate accumulated carbon by inhibiting certain parallel reactions (Eqs. (2), (3)) [21]. However, recent studies indicate that certain types of carbon may produce positive effects on the catalytic reaction [22], and may increase nickel activity and stability in DRM [23], although it is not yet clear which set of factors lead to this behavior. Once the interactions between metal and support influence the type of carbon, the development and optimization of support synthesis has become a promising strategy [24], [25].
Several studies have been carried out using different ceramic materials as supports for nickel catalysts [5], [18], [19], [21]. In these studies, special attention has focused on alumina due to its relative abundance and low cost. Alpha (α) and Gamma (γ) phases of Al2O3 have been used most frequently because of their differentiated structural and textural properties [26], [27]. A critical aspect of the synthesis of Ni/Al2O3 catalysts is the control of metal/support interaction, as well as the formation of NiAl2O4 secondary phases. Ni2+ species dispersed on the surface of γ-Al2O3 diffuse readily, forming NiAl2O4 after calcination above 600 °C [28]. The formation of NiAl2O4 for the DRM reaction should be avoided due to its low reducibility and sintering problems [29], [30]. The high thermal and chemical stability of α-Al2O3 prevents the formation of NiAl2O4 [24] and may favor the production of free NiO on its surface.
For catalytic applications, the properties of the aluminum oxides are improved by adding metals to their structure, forming spinels and perovskites [16], [28]. The addition of lanthanum stabilizes the structure and changes the acid-base properties of γ-Al2O3 due to the formation of perovskite-type LaAlO3 [31], [32]. The typical synthesis of LaAlO3 powders is via solid state reaction by mixing the oxides and calcinating at high temperatures (usually above 1500 °C) [33], [34]. However, this method has some disadvantages, such as the introduction of impurities during the milling process of the precursors, low chemical homogeneity, high reaction temperatures and large particle size of the resulting powders. In recent years, several low-temperature chemical routes have been used to synthesize homogeneous LaAlO3 powders, including the sol-gel [35], Pechini [36], combustion [37], hydrothermal [38], co-precipitation [39], solvothermal [40], combined EDTA-glycine process [41] and sucrose methods [42]. In addition to these methods, the synthesis of monophasic and homogeneous ceramic materials via microwave assisted combustion reaction has been considered simple, fast and inexpensive [43], and was used here to obtain the LaAlO3 structure.
The stable LaAlO3, SrTiO3 and BaTiO3 perovskites have been evaluated as catalytic supports [35], but dry reforming of methane studies using nickel as active metal and LaAlO3 as support are reduced in the literature [28], [44]. In almost a century of research on DRM [14] barely is known about the phenomenological behavior of the Ni/LaAlO3 catalyst. In this context, the purpose of the present study was to synthesize and evaluate the LaAlO3 support in dry reforming of methane using nickel as an active metal. The performance and formation of carbon of the Ni/LaAlO3 catalyst was systematically compared to the Ni catalyst supported on commercial α-Al2O3.
Section snippets
Preparation of the catalysts
The α-Al2O3 powders with an average particle size of less than 74 μm were obtained by grinding the pellets of a 99.00% pure commercial sample (Alfa Aesar). In turn, the LaAlO3 powders were synthesized by microwave assisted combustion, according to procedures adopted for the synthesis of ceramic oxides [43] and based on the general principles of the chemistry of explosives and propellants [45]. All the chemical used in the synthesis of LaAlO3 were from Sigma Aldrich. An excess (20 wt%) of urea
Characterization of the fresh catalysts
According to the XRD patterns shown in Fig. 1, the commercial support contains only the α-Al2O3 crystalline phase with rhombohedral structure, R-3c symmetry (JCPDS card No. 01-089-7716) and an average crystallite size of 59.72 nm (Table 1). LaAlO3 support synthesized by microwave assisted combustion method has characteristic diffraction peaks of the ABO3 perovskite with a rhombohedral structure, R-3c symmetry (JCPDS card No. 01-082-0478) and an average crystallite size of 40.77 nm. The
Conclusions
LaAlO3 monophasic perovskite was successfully obtained by the microwave assisted combustion method. The surface oxygen vacancies of LaAlO3 participated in the NiO reduction mechanism and influenced Ni/support interactions.
The space velocities of H2 applied in the activation affected the activity of Ni/LaAlO3, without altering the performance of Ni/α-Al2O3. The presence of NiO in the reduced catalyst (Ni0/LaAlO3) increased its activity and catalytic stability because of the carbon nanotubes
Acknowledgment
The authors thank the Laboratório de Tecnologia Ambiental (LABTAM-UFRN), PETROBRAS, CENPES, CAPES, CNPq, PPGQ/UFRN and IFMA for financial support and tests, and the Brazilian Synchrotron Light Laboratory (LNLS) for financial support and the use of D10B-XPD beamlines to perform the X-ray diffraction experiments.
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