Liu Hai-Xia, Tian Na, Ye Jin-Yu, Lu Bang-An, Ren Jie, Huangfu Zhi-Chao, Zhou Zhi-You, Sun Shi-Gang
State Key Lab of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, XiamenUniversity, Xiamen, 361005, China.
Faraday Discuss. 2014;176:409-28. doi: 10.1039/c4fd00136b. Epub 2015 Feb 5.
This study focuses on CO adsorption at tetrahexahedral Pt nanocrystals (THH Pt NCs) by using cyclic voltammetry and in situ FTIR spectroscopy. Since the electrochemically prepared THH Pt NCs in this study are enclosed by {730} facets which could be considered by a subfacet configuration of 2{210} + {310}, we have also studied CO adsorption on the interrelated Pt(310) and Pt(210) single crystal electrodes as a comparison. Cyclic voltammetry results demonstrated that CO adsorbs dominantly on the (100) sites of THH Pt NCs at low CO coverage (θ(CO)≤ 0.135), while on both (100) and (110) sites at higher CO coverage. On ordered Pt(310) and Pt(210), i.e. they were flame annealed and then cooled in H(2) + Ar, CO adsorption also illustrates relative priority on (100) sites at low CO coverage; while at high CO coverage or on oxygen-disordered Pt(310) and Pt(210) when they were cooled in air after flame annealing, the adsorption of CO presents a weak preference on (100) sites of Pt(310) and even no preference at all on (100) sites of Pt(210). In situ FTIR spectroscopic studies illustrated that CO adsorption on THH Pt NCs yields anomalous infrared effects (AIREs), which are depicted by the Fano-like IR feature on a dense distribution (60 μm(-2)) and the enhancement of abnormal IR absorption on a sparse distribution (22 μm(-2)) of THH Pt NCs on glassy carbon substrate. Systematic investigation of CO coverage dependence of IR features revealed that, on THH Pt NCs, the IR band center (ν(COL)) of linearly bonded CO (COL) is rapidly shifted to higher wavenumbers along with the increase of CO coverage to 0.184, yielding a fast linear increase rate with a high slope (dν(COL)/dθ(IR)(CO) = 219 cm(-1)); when θ > 0.184, the increase of ν(COL) with θCO slows down and deviates drastically from linearity. In contrast, the ν(COL) on the ordered Pt(310) electrode maintains a linear increase with θ(IR)(CO) for the whole range of θ(IR)(CO) variation, and gives a much smaller increase rate of slope 74.3 cm(-1). The significant differences in CO adsorption behavior on THH Pt NCs and on interrelated Pt single crystal planes demonstrated clearly the unique properties of nanoparticles enclosed by high-index facets.
本研究通过循环伏安法和原位傅里叶变换红外光谱法，聚焦于四十六面体铂纳米晶体（THH Pt NCs）上的CO吸附。由于本研究中通过电化学方法制备的THH Pt NCs被{730}面所包围，该面可视为由2个{210} + {310}组成的子面构型，我们还研究了CO在相关的Pt(310)和Pt(210)单晶电极上的吸附情况作为对比。循环伏安法结果表明，在低CO覆盖度（θ(CO)≤ 0.135）时，CO主要吸附在THH Pt NCs的(100)位点上，而在较高CO覆盖度时则同时吸附在(100)和(110)位点上。在有序的Pt(310)和Pt(210)上，即它们经过火焰退火然后在H₂ + Ar中冷却，在低CO覆盖度时CO吸附在(100)位点上也表现出相对优先性；而在高CO覆盖度时，或者在火焰退火后在空气中冷却的氧无序Pt(310)和Pt(210)上，CO在Pt(310)的(100)位点上的吸附表现出较弱的优先性，在Pt(210)的(100)位点上甚至完全没有优先性。原位傅里叶变换红外光谱研究表明，CO在THH Pt NCs上的吸附产生了异常红外效应（AIREs），这表现为在玻碳基底上密集分布（密度为60 μm⁻²）的THH Pt NCs呈现出类法诺红外特征，以及在稀疏分布（密度为22 μm⁻²）的THH Pt NCs上异常红外吸收增强。对红外特征随CO覆盖度变化的系统研究表明，在THH Pt NCs上，线性吸附的CO（COL）的红外波段中心（ν(COL)）随着CO覆盖度增加到0.184而迅速向高波数移动，产生具有高斜率的快速线性增加速率（dν(COL)/dθ(IR)(CO) = 219 cm⁻¹）；当θ > 0.184时，ν(COL)随θCO的增加减缓并显著偏离线性关系。相比之下，在有序的Pt(310)电极上，ν(COL)在整个θ(IR)(CO)变化范围内随θ(IR)(CO)保持线性增加，且斜率增加速率小得多，为74.3 cm⁻¹。THH Pt NCs与相关Pt单晶平面上CO吸附行为的显著差异清楚地表明了被高指数面所包围的纳米颗粒的独特性质。
