沸石负载铂纳米颗粒上一氧化碳氧化的促进作用。

Promotion effects in the oxidation of CO over zeolite-supported Pt nanoparticles.

作者信息

Visser Tom, Nijhuis T Alexander, van der Eerden Ad M J, Jenken Karin, Ji Yaying, Bras Wim, Nikitenko Sergei, Ikeda Yasuo, Lepage Muriel, Weckhuysen Bert M

机构信息

Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.

出版信息

J Phys Chem B. 2005 Mar 10;109(9):3822-31. doi: 10.1021/jp044767f.

DOI:10.1021/jp044767f

PMID:16851431

Abstract

Well-defined Pt-nanoparticles with an average diameter of 1 nm supported on a series of zeolite Y samples containing different monovalent (H+, Na+, K+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperature-programmed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter alpha. A decreasing alpha results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T(50%) for CO oxidation decreases with decreasing alpha. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T(50%) of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.

摘要

一系列负载有平均直径为1 nm的明确定义的铂纳米颗粒的Y型沸石样品，这些样品含有不同的一价（H⁺、Na⁺、K⁺、Rb⁺和Cs⁺）和二价（Mg²⁺、Ca²⁺、Sr²⁺和Ba²⁺）阳离子，已被用作模型系统来研究促进剂元素在过量氧气中CO氧化反应中的作用。时间分辨红外光谱测量使我们能够研究CO从负载的Pt纳米颗粒上的程序升温脱附，以监测吸附CO的局部环境中的电子变化。结果发现，与气相CO相比，线性Pt配位的CO振动的红移随着阳离子半径与电荷比的增加而增加。此外，观察到随着路易斯酸度降低，CO从线性（L）键合向桥式（B）键合发生系统性转变，如用Kamlet-Taft参数α表示。α的降低导致骨架氧原子上的电子电荷增加，因此负载的Pt纳米颗粒上的电子电荷也增加。这一观察结果通过X射线吸收光谱得到证实，实验Pt原子XAFS的强度与引入阳离子的路易斯酸度相关。此外，发现CO覆盖率随着Pt纳米颗粒上电子密度的增加而增加。发现这种增加的电子密度导致CO氧化活性增加；即，CO氧化的T(50%)随着α的降低而降低。换句话说碱性促进剂促进了CO在Pt颗粒上的化学吸附。促进效果最佳的CO氧化催化剂是Pt/K-Y样品，其T(50%)为390 K，L:B强度比为2.7。所得结果为设计改进的CO氧化催化剂提供了指导。