Pd/Al2O3 catalysts modified with Mg for catalytic combustion of methane: Effect of Mg/Al mole ratios on the supports and active PdOx formation
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摘要: 本文研究了系列不同含量镁助剂改性的Pd/Al2O3催化剂的甲烷催化燃烧反应。研究表明，随着镁添加量的增加，载体由Al2O3转变为尖晶石型MgAl2O4，进一步增加Mg/Al物质的量比至3：1时，形成了Mg（Al）Ox固溶体；催化剂中活性相Pd物种以金属Pd，PdOx或Pd-载体复合氧化物形式存在，各物种的相对含量以及Pd↔PdO间的转化能力存在一定的差异。PdOx物种表现为具有较高的低温活性，而金属Pd和Pd-载体复合氧化物的高温活性较好。当Mg/Al物质的量比为1：3时，催化剂的Pd↔PdO转化能力最强，表现出了最高的甲烷催化燃烧反应活性。Abstract: Pd/Al2O3 catalysts modified by different amount of magnesium were fabricated for catalytic combustion of methane (CCM). After the introduction of different amount of magnesium, Al2O3, MgAl2O4-like mixed oxide and Mg(Al)Ox solid solution were formed. Owing to the formation of distinguished supports, the supported Pd species, i.e. metallic Pd, PdOx and support-Pd oxide complex were formed, and they were quite different in relative content and Pd↔PdO transformation ability. It was found that PdOx was active at low temperature, while metallic Pd particles and support-Pd oxide complex were active at high reaction temperature. The one with Mg/Al mole ratio of 1:3 was the most easily in Pd↔PdO transformation, demonstrating the best catalytic activity towards CCM reaction.
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Key words:
- methane catalytic combustion /
- Pd-based catalyst /
- MgAl2O4 /
- Mg(Al)Ox
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Figure 1 XRD patterns of the (a) Pd/Al2O3, (b) Pd/MgAl4, (c) Pd/MgAl3, (d) Pd/MgAl2 and (e) Pd/Mg3Al catalysts
Figure 2 XRD patterns of catalyst precursors
(a): Pd/MgAl4-p; (b): Pd/MgAl3-p; (c): Pd/MgAl2-p; (d): Pd/Mg3Al-p; -p for precursor
Figure 3 TEM images of the (a) Pd/Al2O3, (b) Pd/MgAl4, (c) Pd/MgAl3, (d) Pd/MgAl2 and (e) Pd/Mg3Al catalysts
Figure 4 O2-TPO profiles of Pd/MgxAlyO catalysts with different Mg/Al mole ratios as a function of temperature during heating (peak up) and cooling (peak down) cycles
Figure 5 H2-TPR profiles of the (a) Pd/Al2O3, (b) Pd/MgAl4, (c) Pd/MgAl3, (d) Pd/MgAl2, and (e) Pd/Mg3Al catalysts
Figure 6 H2-TPR profiles of the Pd/MgAl3 catalyst (a): after oxidizing by 2% O2/He at 500 ℃ for 1 h; (b): purging with He at 400 ℃ for 1 h before reduction
Figure 7 Light-off curves of the Pd/MgxAlyO catalysts with different Mg/Al molar ratios for methane catalytic combustion
Table 1 Physical-chemical properties of the Pd/MgxAlyO catalysts with different Mg/Al mole ratios
Sample Pd loadinga/% Mg/Ala (at.) PdO crystal size b/nm ABETc/ (m2·g-1) thystersisd/ ℃ H2 consumptione/ (μmol·g-1) H2 consumption or PdOxf/ (μmol·g-1) Pd surface areag/ (m2·g-1) Pd/Al2O3 0.96 0 18.3 168 136 11 4.0 - Pd/MgAl4 0.97 1:3.8 15.1 135 144 82 68 46.5 Pd/MgAl3 0.96 1:3.1 15.3 126 119 83 62 56.4 Pd/MgAl2 1.04 1:1.9 13.8 122 154 72 58 67.2 Pd/Mg3Al 0.97 3.2:1 11.9 180 280 9.0 - - a: determined by the ICP-OES analysis;
b: estimated from PdO (101) diffraction peak at ca.39.5° by Scherrer equation;
c: BET surface area calculated from N2 physisorption isotherm;
d: temperature hysteresis (thystersis) between the heating and cooling cycles from TPO profiles in Figure 4, for example, in Pd/Al2O3 catalyst the decomposition of PdOx starts at 710 ℃ during the heating process, while the oxidation of Pd starts at 574 ℃ during the cooling process, so thystersis=(710-574) ℃ =136 ℃;
e: H2 consumption calculated from H2-TPR profiles (sum of peak α and β);
f: H2 consumption calculated from H2-TPR profiles (subtracting H2 consumption of peak α and β to that of peak γ);
g: Pd surface area derived from the H2-O2 titration experiment
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