Nickel-based catalysts show excellent activity towards C-C, C-O and C-H bond cleavage, therefore, because of its lower price and higher abundance compared to noble metals, these materials are widely employed in syngas generation from methane reforming processes [1]. However, nickel catalyst shows high deactivation rates due to coke deposition [2]. In this context, perovskite-type precursors appear as a valid solution to this problem, due to their particular ability to produce nickel dispersed particles over an oxide support after H2 pre-treatment, increasing catalyst stability [3]. Many studies on the literature have reported that the nature of A and B-site cation on perovskites strongly influences its catalytic properties. Particularly, cerium and strontium substituted lanthanum-nickel perovskites have shown interesting properties that worth deeply investigation. In this sense, this work aims to study the effect of Ce, La and Sr over nickel-based catalyst precursors on Oxy-dry reforming.
2. Experimental.
Perovskite precursors MNiO3 (M= La, Sr and Ce) were prepared using citrate method and calcined at 1073°C for 5h. After that, the materials were characterized using temperature-programmed reduction (TPR), X-ray diffraction (XRD), in-situ X-ray diffraction at reducing atmosphere (XRD-H2), Temperature-programmed surface reaction (TPSR) and Thermo-programmed CH4-cracking reaction (TPCR-CH4). Oxy-dry reaction was conducted in a tubular quartz reactor at 1073K during 15h, with 15 mg of catalyst, under a continuous feed of 70 mL.min-1 in a CH4/CO2/O2 4:2:1 flow. The samples were pretreated under H2 atmosphere (30 mL.min-1) at 1073 K during 1h before TPSR and TPCR experiments.
3. Results and Discussion.
The XRD and TPR results, for the samples, are shown in Figure 1.
Figure 1: X-ray diffraction (Left) and Thermo Programmed Reduction (Right) profiles of the samples a) LaNiO3 b) CeNiO3 c) SrNiO3. ♥ Sr2Ni2O5; ♦ CeO2; ♣ SrCO3; • NiO; ♠ LaNiO3
The XRD results have shown that the sample LaNiO3 presented the formation of perovskite phase (ICDD-01-079-2451), along with NiO segregate (ICDD-00-002-1216) while, the precursor CeNiO3 were composed basically by CeO2 (ICDD-03-065-5923) and NiO (ICDD- 01-075-0269) oxides. Meanwhile, the perovskite SrNiO3 exhibited a complex diffraction profile, with lines relative to SrCO3 (ICDD-00-005-0418), NiO (ICDD-01-078-0423) and Sr2Ni2O5 (ICDD-00-028-1242). The results indicated a low solubility of Ce and Sr cations in the perovskite MNO3 structures, since the first one stabilizes as Ce4+ in the fluorite structure CeO2 because of the high calcination temperatures employed in the perovskite synthesis [4]. While the latter is thermodynamically more stable as a +2 cation than a +3 cation and tends to form carbonates in presence of organic matter, like citrates.
In fact, the presence of these phases is in accordance with the TPR profiles of the samples. In the case of the reaction using LaNiO3, three-steps can be clearly observed, relative to the sequential reduction of Ni3+ to Ni2+ followed by Ni2+ to Ni0. An intermediary peak was attributed to NiO, which was reduced to Ni0, consuming H2. Similarly, the SrNiO3 sample showed three H2 consumption events. Both were attributed to Ni2+ reduction to Ni0 at lower temperature, being the first attributed to Ni2+ from NiO phase, and the later to the decomposition of Sr2Ni2O5 to Ni0 and SrO. The last peak, according to in-situ XRD studies is relative to SrCO3 reduction, generating SrO and CO2. On the other hand, CeNiO3 presented a reduction in two steps due to the reduction of NiO and partial reduction of Ce-based oxides.
Figure 2. Thermo programmed superficial oxy-dry reactions profiles for a) LaNiO3; b) SrNiO3; c) CeNiO3.
Previously the TPSR analyses (Fig. 2), methane cracking tests were performed. The presence of cerium seemed to decrease the activation energy of C-H bond cleavage, leading to lower reaction temperatures, while the strontium addition has promoted an activity decrease. Indeed, the Oxy-CO2 reactions (1 atm, 1073 K, 15 h), after H2-reduction pretreatment, were according to these results and the methane and carbon dioxide conversions had the following decreasing order: CeNiO3 > LaNiO3> SrNiO3, which can be attributed to the possible coverage of Ni0 sites by SrCO3 [4] detected via XRD and TPR analyses. No deactivation was observed over CeNiO3, possibly due to the inherent oxygen mobility of Ceria-based catalysts [4].
TPSR studies indicated a reaction scheme with an initial combustion-reforming of methane, followed by dry reforming of methane leading to the formation of syngas. The ignition temperature of the combustion following the order: CeNiO3 > LaNiO3 > SrNiO3. This indicates the superior ability of cerium oxides to promote oxygen-based reactions. Moreover, CeNiO3 presented also a short combustion step, almost instantly followed by the dry reforming reaction, comparing with LaNiO3. This may be associated with the higher reducibility of NiO in relation with the perovskite oxides. The selectivity to H2 and CO were not affected by the A-site perovskite substitution. Deactivation of the catalysts was not observed after 15 h on-stream.
4. Conclusions.
XRD studies indicated the presence of perovskite-type structure only on La-containing precursor. Instead, the other two precursors revealed by XRD a mixture of segregate solids. All materials were active to syngas production and the A-site substitution caused significant variation on C-H cleavage activation rate, following the order: CeNiO3 > LaNiO3> SrNiO3. No significant deactivation by carbon deposition was verified after 15h on stream.
5. References.
[1] Xue Z., Shen Y., Zhu, S. Promoting effects of lanthanum oxide on the NiO/CeO2 catalyst for hydrogen production by autothermal reforming of ethanol. Catal. Comm. 108 (2018) 12-16.
[2] Castro T., Rabelo C., Noronha F. Study of the performance of Pt/Al2O3 and Pt/CeO2/Al2O3 catalysts for steam reforming of toluene, methane and mixtures. Catal. T. (http://dx.doi.org/10.1016/j.cattod.2017.05.067).
[3] Santos, M. Noronha, F., Brandao, S. Perovskite as catalyst precursors in the partial oxidation of methane: The effect of cobalt, nickel and pretreatment. Catal. T. 299, (2018) 229-241.
[4] Yang E., Noh Y., Perovskite as catalyst precursors in the partial oxidation of methane: The effect of cobalt, nickel and pretreatment. Catal. T. (http://dx.doi.org/10.1016/j.cattod.2017.03.050)
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