氧化锌纳米线外延生长PdZn纳米粒子的制备及在甲醇水蒸气重整反应中的催化性能

doi:10.7503/cjcu20160061

摘要/Abstract

摘要：

以采用改进的气相沉积法制备的具有规整{10 $1 ˉ$ 0}晶面的氧化锌纳米线为载体, 合成了氧化锌纳米线负载钯催化剂, 考察了还原温度和负载量对催化剂表面形成PdZn合金过程的影响, 并通过适当的后处理过程制备了氧化锌纳米线外延生长PdZn纳米粒子催化体系. 结果表明, 当金属钯负载量较低(质量分数约为2%)时, 经400 ℃还原后的催化剂表面会形成Pd_xZn_y(x>y)合金, 从而影响催化剂的CO选择性; 提高钯负载量或还原温度有利于将Pd_xZn_y(x>y)合金转化为PdZn合金, 降低CO选择性. 负载PdZn合金纳米粒子与氧化锌纳米线载体之间外延生长的界面关系使其在甲醇水蒸气重整反应中显示出优异的反应稳定性.

关键词: 氧化锌纳米线, 钯, 外延生长, 负载型催化剂, 甲醇水蒸气重整

Abstract:

ZnO nanowires (NWs) supported Pd catalysts were prepared by modified deposition-precipitation method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), transmission electron microscopy (TEM) and high angle annular dark field scanning transmission electron microscope (HAADF-STEM). The effects of Pd loading and reduction temperature on the formation of PdZn were investigated. The results showed that the high CO selectivity of 2.1%Pd/ZnO NW catalyst after reduced at 400 ℃ is due to the formation of Pd_xZn_y(x>y) alloy. Increasing of Pd loading or reduction temperature is favorable for the transformation of Pd_xZn_y(x>y) to PdZn alloy which can inhibit CO formation. By a modified wet chemistry method and apt post-synthesis treatment, we have been able to grow PdZn alloy nanoparticles epitaxially onto the {10 $1 ¯$ 0} nanofacets of the ZnO NWs. It showed better stability than PdZn supported on commercial ZnO powders in long term reaction. The methanol conversion at 300 ℃ over Pd/ZnO NW remained constant for about 40 h without obviously deactivation. On the contrary, the methanol conversion over Pd/ZnO powders catalyst gradually decreased with time on stream by more than 30%. The better stability of Pd/ZnO NW catalysts is due to the highly stable ZnO NWs primarily enclosed by the low-energy {10 $1 ¯$ 0} surfaces and the epitaxial relationships between PdZn nanoparticles and ZnO NWs support.

Key words: ZnO nanowires, Palladium, Epitaxial growth, Supported catalyst, Methanol steam reforming

中图分类号:

O643.3

TrendMD:

刘佳欣, 宋义安, 黄玉东. 氧化锌纳米线外延生长PdZn纳米粒子的制备及在甲醇水蒸气重整反应中的催化性能. 高等学校化学学报, 2016, 37(7): 1372.

LIU Jiaxin, SONG Yian, HUANG Yudong. PdZn Alloy Nanoparticles Epitaxial Growth on ZnO Nanowires for Methanol Steam Reforming^†. Chem. J. Chinese Universities, 2016, 37(7): 1372.

图/表 10

Fig.1 XRD patterns of 4.9%Pd/ZnO NW catalysts after reduced at different temperatures(A) and Pd/ZnO NW-400 catalysts with different Pd loadings(B) (A) a. Unreduced; b. 100 ℃; c. 200 ℃; d. 300 ℃; e. 400 ℃; f. 500 ℃; g. 600 ℃; (B) a. ZnO NW; b. 2.1%; c. 4.9%; d. 10.0%.

Fig.2 Pd3d XPS spectra of 2.1%Pd/ZnO NW catalysts after reduced at different temperatures (A) 100 ℃; (B) 200 ℃; (C) 300 ℃; (D) 400 ℃; (E) 500 ℃.

Fig.3 TEM images(A—C) and corresponding particle size distributions(A'—C') of Pd/ZnO NW-H400 catalysts with different Pd loadings (A, A') 2.1%; (B, B') 4.9%; (C, C') 10.0%.

Fig.4 HAADF-STEM image(A), area-indexed images of Fourier transforms(B—D) and Fourier filtered image(E) of 2.1%Pd/ZnO NW-400 catalyst (B) Pd2Zn[111]; (C) ZnO[112ˉ0]; (D) PdZn[100].

Fig.5 HAADF-STEM image(A), fast Fourier transforms pattern(B) and Fourier filtered image of 4.9%Pd/ZnO NW-400 catalyst (C)

Fig.6 Schematic diagram of PdZn alloy nanoparticles epitaxial growth on ZnO{101ˉ0} surface (A) Deformation in epitaxial layers; (B) formation of misfit dislocation at interface.

Fig.7 Influence of reduction temperature on the steam reforming of methanol over Pd/ZnO NW catalysts with different Pd loadingsPd loading(mass fraction): a. 2.1%; b. 4.9%; c. 10.0%. (A) Conversion; (B) CO selectivity.

Fig.8 Stability test of 4.9%Pd/ZnO NW-400 and 5.1%Pd/ZnO P-400 catalysts at 300 ℃ in methanol steam reforming reaction

Fig.9 TEM images(A—C) and PdZn size distributions(A'—C') of Pd/ZnO NW-400-used(A, A'), Pd/ZnO P-400(B, B') and Pd/ZnO P-400-used catalysts(C, C')

Fig.10 HRTEM images of Pd/ZnO P-400 catalyst

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