Steam reforming of ethanol over Ni-based catalysts obtained from LaNiO3 and LaNiO3/CeSiO2 perovskite-type oxides for the production of hydrogen
Graphical abstract
Introduction
Recently, ethanol has been claimed to be an attractive fuel for hydrogen production, since it can be manufactured from biomass and thus not contribute to net CO2 emissions. Moreover, in countries like Brazil and the USA, ethanol production and distribution infrastructure is already established [1].
Hydrogen may be generated from ethanol by steam reforming (SR) (Eq. (1)) [2], [3], [4], [5], [6], [7], [8]. However, one of the main barriers of the SR of ethanol technology is the catalyst deactivation that occurs mainly due to carbon deposition on catalyst surface, which may lead to a decrease in catalytic activity and selectivity toward hydrogen [2], [3], [4], [5], [6], [7], [8].C2H5OH + 3H2O → 6H2 + 2CO2
Carbon formation may take place via several reactions, such as ethanol dehydration to ethylene (Eq. (2)), followed by polymerization to coke (Eq. (3)); the “Boudouard” reaction (Eq. (4)); the decomposition of methane (Eq. (5)) [6], [7]. The extent of each reaction depends on both reaction conditions and catalyst used. Therefore, the development of a catalyst resistant to carbon deposition during SR of ethanol is one of the main issues of this technology.C2H5OH → C2H4 + H2OC2H4 → polymers → coke2CO ↔ CO2 + CCH4 ↔ 2H2 + C
Ni-based catalysts have been extensively studied for the SR of ethanol due to their low cost and high activity but this metal is prone to coking [9], [10], [11].
In order to suppress carbon deposition, different approaches have been used, such as: (i) controlling the nickel crystallite size and (ii) using redox supports [6].
The nickel particle size significantly influences the nucleation rate of carbon. The initiation step for carbon formation is more difficult for smaller particle sizes [12]. A critical ensemble size (ensembles of 6–7 atoms) was proposed, below which carbon formation does not occur.
Several works in the literature [11], [13], [14], [15], [16], [17], [18], [19] reported that perovskite-type oxides are promising precursor catalyst for SR of ethanol. The reduction of this mixed oxide produces thermally stable and highly dispersed metallic particles [20]. However, the high reduction temperatures required for the removal of metal from the perovskite structure leads to the generation of large metallic Ni particles, which limits the effectiveness of this strategy [21]. One alternative is to deposit the perovskite-type oxides over a high surface area support. Different supported perovskite-type oxides have been reported in the literature for several reactions such as CO oxidation (LaCoO3/Al2O3 [21]; LaMO3/ZrO2, M = Ni, Co, Fe [22]; LaCo0.5M0.5O3/cordierite [23] LaCoO3/ZrO2 [24]), hydrocarbon combustion (LaMnO3/MgO [25]; LaMnO3/ZrO2 [26]; LaMnO3/Al2O3 [27]; LaFeO3/Al2O3 [28]), steam reforming of methane (LaNiO3/Al2O3 and LaNiO3/ZrO2 [29]), conversion of tar (LaCoO3/Al2O3 [30]), NOx reduction (LaCoO3/K2CO3/CeO2 [31]). However, there is no work in the literature that studies the performance of supported perovskite-type oxides for SR of ethanol.
The type of support affects the product distribution and catalyst stability during ethanol conversion reactions since it also exhibits activity for this reaction. In the case of redox supports like ceria or ceria-mixed oxides, their high oxygen mobility promotes the mechanism of carbon removal, which in turn contributes to the high stability of the catalysts on ethanol conversion reactions [6], [32]. Doping ceria with silica not only improves oxygen mobility but also inhibits the sintering of CeO2 crystallites [33].
Therefore, the aim of this work is to study the performance of Ni-based catalysts prepared from LaNiO3 and CeSiO2 supported LaNiO3 perovskite-type oxide for the production of hydrogen through SR of ethanol at different temperatures. In situ X-ray diffraction analyses (XRD) were also performed, which allowed to monitor changes on the catalyst structure during the reduction and SR of ethanol reaction.
Section snippets
Catalysts preparation
The perovskite-type oxide was prepared by combustion method, using a fuel-to-oxidizers ratio (φ) of 0.7, which was defined as the ratio of the total valences of fuel (urea) to the total valence of oxidizers (nitrates of nickel and lanthanum) [10]. The combustion method exhibits several advantages such as low processing cost, simplicity and high production rate. Furthermore, this method is appropriated for the preparation of monolithic catalysts by dip-coating. The aqueous suspension containing
Catalyst characterization
The Ni, La, Ce and Si content of all samples obtained by XRF analysis were showed in Table 1. The Ni loading was close to the nominal value for both samples studied (LaNiO3: 23.0 wt% Ni; LaNiO3/CeSiO2: 6.9 wt% Ni). The BET surface area of unsupported LaNiO3 was very low (<10.0 m2/g), which is characteristic of these materials prepared after calcination at high temperature [10]. The BET surface area of CeSiO2 was 51 m2/g. In general, CeO2 materials calcined at high temperature (1073 K) exhibit very
Conclusions
The results obtained showed that LaNiO3/CeSiO2 is a promising catalyst for H2 production from ethanol, since this material exhibited high activity and lower carbon formation during SR of ethanol at 773 K.
In situ XRD experiments revealed that the deposition of LaNiO3 over a ceria oxide promoted Ni dispersion and inhibited the oxidation of metallic Ni crystallites during SR of ethanol at 773 K. Furthermore, ceria support was also reduced, promoting the formation of oxygen vacancies. The smaller
Acknowledgements
The authors acknowledge the scholarship received from FAPERJ. The group thanks the LNLS for the assigned time at XPD-10B beamline.
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