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Abstract: Perovskites as host structures of cations were used in order to generate in situ active and stable catalysts for ethanol steam reforming. For this purpose, La1-xMgxAl1-yNiyO3 (x=0.1; y=0, 0.1, 0.2, 0.3) perovskites were synthetized by the citrate method. Ni segregation is evident for a substitution level higher than 0.2. The segregation of Ni as NiO generated species interacts with different metal-support after the reduction step. The y=0.1 catalyst presents the highest H2 yield value about 85% during reaction time, with low mean values of CH4 and CO selectivities of 3.4% and 11%, respectively and a low carbon formation. The better performance of y=0.1 catalyst could be attributed to the minor proportion of segregated phases, thus a controlled expulsion of Ni is successfully reached.
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Key words:
- perovskites /
- catalyst design /
- ethanol /
- steam reforming
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Figure 1 Catalytic performance of perovskite catalysts during 9 h on stream
● : ethanol conversion; ▲ : H2 yield; : CO selectivity; ◇ : CO2 selectivity; ■ : CH4 selectivity
Figure 2 TG results of catalysts used in ethanol steam reforming reaction
a: LaMgNi1; b: LaMgNi2; c: LaMgNi3
Figure 6 Neutron powder diffraction Rietveld profiles of LaMgNiy for (a) y=0.1, (b) y=0.2 and (c) y=0.3
Figure 7 A view of the crystal structure of LaMgNi2 at RT, defined as a superstructure of perovskite consisting of a framework of slightly tilted (Al, Ni)O6 octahedra with La atoms in the voids
Table 1 BET specific surface area, H2 consumption from TPR, Ni content from ICP-OES and weight loss from TG
Catalyst ABET/(m2·g-1) H2 consumption/(μmol·mgNi-1) Ni content w/% Weight loss w/% LaAlO3 14 - - La0.9Mg0, 1AlO3 15 - - NiO/LaAlO3 17 0.63 3.3 (2.8)* LaMgNi1 13 2.16 2.2 (2.8) 6 LaMgNi2 10 2.04 3.4 (5.6) 7 LaMgNi3 8 1.39 5.2 (8.3) 15 *: (values between brackets correspond to nominal ones) 
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Table 2 Structural parameters for the perovskites with nominal composition La0.9Mg0.1Al1-yNiyO3, refined in the rhombohedral R3c (no. 167) space group at 25 ℃ from NPD data
Ni contents y= 0.1 y= 0.2 y= 0.3 a/Å 5.3670(6) 5.3735(5) 5.3821(5) c/Å 13.139(2) 13.149(2) 13.149(2) v/Å3 327.75(8) 328.79(7) 329.86(6) La/Mg 6a (0 0 1/4) focc La/Mg 0.99(2)/0.01(2) 1.00(2)/0.00(2) 0.99(2)/0.01(2) B/Å2 0.36(4) 0.36(3) 0.31(3) (Al, Ni) 6b (0 0 0) focc Al/Ni 0.95(1)/0.05(1) 0.936(4)/0.064(1) 0.810(2)/0.190(2) B/Å2 0.49(6) 0.47(5) 0.88(3) O 18e (x 0 1/4) x 0.4751(2) 0.4737(2) 0.4717(2) focc 1.0 1.0 1.0 B/Å2 0.69(3) 0.77(2) 0.88(3) Fraction main phase 96.6(1) 93.8(1) 93.1(1) Fraction MgO 1.2(1) 0.6(1) 2.7(1) Fraction NiO 2.2(1) 5.6(1) 4.2(1) Reliability factors χ2 2.20 5.22 2.22 Rp/% 2.82 2.60 2.82 Rwp/% 2.27 3.61 2.12 RI/% 2.48 1.58 1.77
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Table 3 Main interatomic distances (Å) and selected angles (°) for the perovskites with nominal composition La0.9Mg0.1Al1-yNiyO3
Ni content y=0.1 y= 0.2 y= 0.3 (Al, Ni)-O ×6 1.9018(7) 1.9044(7) 1.9073(8) O-(Ni, Mo)-O 90.16(4) 90.20(4) 90.28(5) (Ni, Mo)-O-(Ni, Mo) 171.94(2) 171.50(2)
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Table 4 XPS results of catalysts
Catalyst n(Ni)
/n(La+Mg+Al)n(Mg)
/n(La+Ni+Al)n(Oad)
/n(Ol)LaMgNi1 0.048 (0.05)* 0.05 (0.05) 0.15 LaMgNi2 0.091 (0.11) 0.07 (0.05) 0.18 LaMgNi3 0.131 (0.176) 0.095 (0.05) 0.22 *: values between brackets correspond to nominal ones
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