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摘要: 以MWCNTs为载体，用HNO3做氧化剂对MWCNTs进行处理，通过XPS研究了处理前后MWCNTs表面官能团的变化，并采用超声浸渍法制得Pd/MWCNTs催化剂，借助TEM，揭示了Pd粒子在催化剂表面分散度和粒径与MWCNTs表面含氧量、羟基和羰基间的关系。并考察了Pd/MWCNTs催化剂预处理方法对低浓甲烷催化燃烧活性和稳定性的影响，研究表明，Pd的价态以及Pd粒子的粒径，都与催化剂的甲烷燃烧活性直接关联。单质Pd的氧化和Pd粒径的长大是导致催化剂性能衰减的原因。通过原位FT-IR技术对反应气氛下中间物种的监测，提出了Pd/MWCNTs催化剂上的低浓度甲烷催化氧化反应机理。Abstract: The raw multi-walled carbon nanotubes (MWCNTs) were treated with nitric acid. The shift of the surface functional groups on the MWCNTs was observed with XPS. The Pd/MWCNTs catalysts were synthesized by the ultrasonic impregnation method. The total contents of oxygen, hydroxyl and carbonyl groups were measured. The dispersion and size distribution of Pd particles were characterized with TEM. The dependence of Pd dispersion on the oxygen-containing functional groups was validated. The effect of pretreatment on the catalytic activity and stability for methane combustion was investigated under lean fuel conditions. It is shown that the catalytic activity depends on the valence state and particle size of palladium. The transformation from Pd to PdO possibly caused the decrease in the catalytic activity. Another factor inducing deactivation is Pd particle aggregation. The reaction mechanism for methane combustion over the Pd/MWCNTs catalyst is postulated on the basis of the intermediate species detected by in-situ FT-IR spectroscopy.
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Key words:
- MWCNTs /
- methane combustion /
- oxygen-contained functional groups /
- active species /
- reaction mechanism
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图 2 催化剂和载体的C 1s和O 1s XPS谱图
Figure 2 Experimental and simulated XPS of C 1s (1) and O 1s (2) of MWCNT-un (a), MWCNT-as (b) and Pd/MWCNT-A (c)
图 3 Pd/MWCNT-un和Pd/MWCNT-D的TEM照片
Figure 3 TEM images and Pd particle size distributions of Pd/MWCNT-un (a, b, c) and Pd/MWCNT-D (d, e, f)
图 4 不同方法制得的Pd/MWCNTs催化剂的活性
Figure 4 Catalytic results of Pd/MWCNTs samples for combustion of methane
图 5 催化剂中Pd的XPS谱图
Figure 5 Experimental and deconvoluted Pd3d XPS of Pd-MWCNT-A (a) and Pd-MWCNT-D (b)
图 6 Pd/MWCNT-A和Pd/MWCNT-D催化剂的TEM照片
Figure 6 TEM images of Pd/MWCNT-A (a) and Pd/MWCNT-D (b)
图 7 10%CH4-90%Ar条件下Pd/MWCNTs催化剂上的原位红外光谱谱图
Figure 7 In-situ FT-IR spectra of Pd/MWCNTs catalysts in 10% CH4-90% Ar atmosphere
图 8 30%O2-70%Ar气氛下Pd/MWCNTs催化剂上的原位红外光谱谱图
Figure 8 In-situ FT-IR spectra of Pd/MWCNTs catalysts in 30% O2-70% Ar atmosphere
图 9 10%CH4-30%O2-60%Ar气氛下Pd/MWCNTs催化剂上的原位红外光谱谱图
Figure 9 In-situ FT-IR spectra of Pd/MWCNTs catalysts in 10% CH4-30% O2-60%Ar atmosphere
图 10 Pd/MWCNT-D催化剂在低浓度甲烷燃烧反应中的稳定性
Figure 10 Methane conversion obtained over the Pd/MWCNT at different time for combustion of methane at GHSV of 40 000 h-1
(0.4% CH4, air balance, 330 ℃)
图 11 Pd/MWCNT-D催化剂失活后的TEM照片
Figure 11 TEM images for the spentPd/MWCNT-D
(a), (b), (c): bright field images; (d): selected area electron diffraction pattern
表 1 催化剂表面C和O元素组成
Table 1 Surface content of C and O for MWCNTs and Pd/MWCNTs samples
Sample Relative content of C /% Relative content of O /% MWCNT-un 0.95 0.05 MWCNT-as 0.93 0.07 Pd/MWCNT-A 0.81 0.19 Pd/MWCNT-B 0.93 0.07 
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表 2 催化剂及载体中官能团含量
Table 2 Surface chemical compositions of different functional groups in the MWCNT-un, MWCNT-as and Pd/MWCNT-A
Sample Relative content C=O -OH others MWCNT-un 0.51 0.36 0.13 MWCNT-as 0.44 0.46 0.10 Pd/MWCNT-A 0.37 0.47 0.16
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表 3 样品中不同Pd物种的含量
Table 3 Surface content of Pd species in the Pd-MWCNT-A and Pd-MWCNT-D
Sample Relative content Pd0 PdO (Pd2+) Pd/MWCNT-A 0.23 0.77 Pd/MWCNT-D 0.76 0.24
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