X-ZrO2 addition (X= Ce, La, Y and Sm) on Ni/MgAl2O4 applied to methane tri-reforming for syngas production
Introduction
CO2 utilization is one of the paths among the CCSU (CO2 Capture, Storage and Utilization) policies to control this pollutant emission to the atmosphere [1]. The methane reforming with the CO2, represented by the Eq. (1) (MDR – Methane Dry Reforming), is a conversion pathway to transform a low cost carbon source, as CO2, into a more valuable products, as fuels and chemicals [2]. The synthesis gas (H2+CO) produced in MDR reaction can be used as raw material to the Fischer-Tropsch process to obtain dimethyl ether, methanol and synthetic fuels and thus substituting the diesel oil and gasoline [3]. During their studies about MDR, Fischer and Tropsch, in 1928, observed the severe Ni-Co catalyst deactivation due to carbon (coke) deposits formation on its surface [3].
The process known as Methane Tri-Reforming (MTR) combines the MDR with the Methane Steam Reforming (MSR- Eq. (2)) and Partial Oxidation (POM- Eq. (3)) of CH4. The main advantages of these combined reactions over each reaction occurring separately are the decrease of the carbon deposition, and higher energetic efficiency due to the exothermic POM, that supplies part of the energy required by the endothermic MDR and MSR reactions. Moreover, more appropriate syngas composition to Fischer-Tropsch synthesis can be achieved using the MTR approach, once MDR generates a syngas whose H2/CO ratio is close to 1, while the required ratio for the hydrocarbons production by means of FT processes is close to 2. By manipulating the composition of the feed, the H2/CO = 2 can be achieved during the MTR [4,5].
Zirconia as a support for catalysts whose active phase is Ni0 has been extensively investigated for MTR application [[6], [7], [8]] because of its resistance to coke [9]. Some disadvantages of the use of zirconia in reforming reactions at elevated temperature are its low specific surface area and the area loss during the reaction due to support sintering, besides its weak mechanical stability [9,10]. In order to avoid these issues, zirconium oxide can be deposited on high surface area materials, for instance, MgAl2O4 spinel.
MgAl2O4 is known to offer resistance to the catalyst towards carbon deposition reactions. It also features basicity properties and is less expensive than ZrO2, reducing the cost of the catalyst [10]. Zirconia is usually associated to lanthanides and alkaline elements aiming at the transformation from the monoclinic phase, which is unstable, into the stable cubic and/or tetragonal phase. The effects of this association are the increase of the oxygen vacancies and NiO reducibility, as well as the zirconia physical properties improvement [9]. A previous work [11] also showed that MgAl2O4 promoted with Zr and Ce is suitable as a support for the nickel catalyst during the TRM reaction.
The objective of the present work is to study nickel catalysts supported on (X-ZrO2)/MgAl2O4 (X = Ce, La, Yor Sm) and investigate the effects of the zirconia association to these elements (“X”) on the TRM.
Section snippets
Catalysts syntheses
MgAl2O4 was synthesized by means of P123® assisted coprecipitation methodology. All the details about the synthesis protocol were described previously [11]. Zr and X + Zr (X/Zr molar ratio of 0.25, X = Ce, La, Y or Sm) were impregnated on the support by the incipient impregnation technique (1.11 X + Zr mmol/g MgAl2O4), using an aqueous solution of ZrO(NO3)2.6H2O, Ce(NO3)3.9H2O, La(NO3)3.6H2O, Y(NO3)3.6H2O and Sm(NO3)3.6H2O. They were purchased from Aldrich. After impregnating, the supports were
X-ray diffraction
The MA (MgAl2O4) support exhibits spinel structure and periclase (MgO) as an additional phase, detected by the shoulders at 2θ = 43° and 62°, after calcination at 750 °C (Fig. 1) [14]. According to Mosayebi and coworkers [15], spinel can be formed at temperatures higher than 600 °C. This occurs once MgO produced from brucite (Mg(OH)2) decomposition reacts directly with Al(OH)3, that is stable up to 600 °C.
Cubic and/or tetragonal zirconia was observed as a secondary phase in the fresh catalysts
Conclusions
Ce- ZrO2 association on the spinel support showed positive effects, leading to an increase of CH4 and CO2 conversions and highest H2 and CO yields. This is due to its smallest apparent nickel particle size and also due to the greater concentration (in percent) of the non-strong natured base sites that made this catalyst adequate to be applied in the tri-reforming of methane reaction. The base sites up to moderate strength (non-strong base sites), ascribed to the hydroxyls and Mn+-O−2 pairs, may
Declaration of Competing Interest
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All authors have participated in
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conception and design, or analysis and interpretation of the data;
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drafting the article or revising it critically for important intellectual content; and
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approval of the final version.
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This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
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The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
Acknowledgements
The authors are thankful to the São Paulo Research Foundation -FAPESP (grant 2014/25972-8 and proc. 2015\06246-7) for the institutional support and studentship, Shell Brazil and FAPESP through the Research Centre for Gas Innovation (grant 2014/50279-4), the Brazilian National Synchrotron Light Laboratory (LNLS) for the XANES experiments (Proposal 2017-0828), and also to the Professor Dr. Tiago Venâncio (Chemical Department of UFSCar) for the 27Al-RMN characterizations. This study was also
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