PdZn Alloy Nanoparticles Epitaxial Growth on ZnO Nanowires for Methanol Steam Reforming†

doi:10.7503/cjcu20160061

Abstract

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

CLC Number:

O643.3

TrendMD:

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

Figures/Tables 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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