Surface reactivity of Pd nanoparticles supported on polycrystalline substrates as compared to thin film model catalysts:: Infrared study of CO adsorption
Surface reactivity of Pd nanoparticles supported on polycrystalline substrates as compared to thin film model catalysts:: Infrared study of CO adsorption
复制标题
DOI：
10.1021/jp036718t
复制
发表时间：
2004-03-18
影响因子：
3.3
通讯作者：
Freund, HJ
中科院分区：
文献类型：
--
作者：
Bertarione, S;Scarano, D;Freund, HJ
为了确定负载型催化剂纳米颗粒表面的性质和局部结构，我们对两类氧化物负载型钯催化剂的CO吸附进行了详细的比较研究：(1)采用浸渍技术制备的多晶MgO和γ - al2o3负载型钯金属催化剂，具有不同程度的规律性和完善性；(2)在超高真空（UHV）条件下制备的单晶Pd模型催化剂。将CO振动频率分配到这些系统上不同类型的位置，可以进行详细的结构表征。对于Pd模型催化剂，在低CO覆盖下，红外（IR）反射吸收光谱与预期的终止行为非常相似，大部分（111）面和少数（100）面。光谱特征表明缺陷的位置，如粒子台阶和边缘。缺陷位置的占用可能受到表面污染（如原子碳）的影响。因此，在高覆盖的CO光谱可以用作反应条件下的结构和化学探针，前提是可以获得关于颗粒形态的补充信息。对于MgO和γ - al2o3支持的Pd体系，在Pd(100)/(111)边缘或切面上，线性键合和桥键合的CO物质有两个明显的窄带（v等于2070 cm和1970 cm(-1)），这与之前在模型催化剂上得到的结果一致。2070厘米（-1）的宽特征表明（100）和（111）面同时存在，边缘和角点出现在它们的交叉处。在高温处理的Pd/MgO样品上，在1970 cm（-1）处有高强度和小半宽（fwhm），这表明金属颗粒暴露的表面具有高度的规律性。进一步的光谱特征（v等于1920-1800 cm(-1)）归因于（111）表面存在不同类型的3倍空心点。
To identify the nature and the local structure of the surface of supported catalyst nanoparticles, we have performed a detailed comparative study of CO adsorption on two categories of oxide-supported Palladium catalysts: (1) polycrystalline MgO and gamma-Al2O3 supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal based Pd model catalysts prepared under ultrahigh vacuum (UHV) conditions. The assignment of the CO vibrational frequencies to different types of sites on these systems has allowed a detailed structural characterization. For the Pd model catalyst, at low CO coverage, the infrared (IR) reflection absorption spectra closely resemble the expected behavior for terminations by a majority of (111) facets and a minority of (100) facets. The spectral features are indicative of defect sites such as particle steps and edges. Occupation of the defect sites can be affected by surface contaminations such as atomic carbon. Thus the CO spectra at high coverage can be used as both a structural and chemical probe under reaction conditions, provided that complementary information on the particle morphology is available. For the MgO and gamma-Al2O3 supported Pd systems, two distinct narrow bands (v congruent to 2070 and congruent to 1970 cm(-1)) have been assigned to linearly bonded and bridge-bonded CO species, on Pd (100)/(111) edges or facets, in agreement with the previous results obtained on model catalysts. The broad character of the 2070 cm(-1) feature indicates the simultaneous presence of (100) and (111) faces, with edge and corner sites present at their intersection. The high intensity and the small half-width (fwhm) of the band at 1970 cm(-1) on a Pd/MgO sample treated at high temperature, assigned to bridge-bonded CO species, suggests that the metal particles expose faces with a high level of regularity. Further spectroscopic features (v congruent to 1920-1800 cm(-1)), are ascribed to the presence of different types of 3-fold hollow sites on (111) faces.