Rioux Robert M, Hoefelmeyer James D, Grass Michael, Song Hyunjoon, Niesz Krisztian, Yang Peidong, Somorjai Gabor A
Department of Chemistry, University of California-Berkeley, CA 94720, USA.
Langmuir. 2008 Jan 1;24(1):198-207. doi: 10.1021/la702685a. Epub 2007 Dec 6.
The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 1.7 to 7.1 nm by diffuse-reflectance infrared spectroscopy and chemisorption. Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure. Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C-O stretch: nu(CO) = 2075 cm-1 (1.9 nm); 2079 cm-1 (2.9 nm); 2082 cm-1 (3.6 nm); and 2090 cm-1 (7.1 nm). The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providing semiquantitative surface coverage information. Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopic features at approximately 1340, approximately 1420, and approximately 1500 cm-1 assigned to ethylidyne, di-sigma-bonded ethylene, and pi-bonded ethylene, respectively. The ratio of these surface species is highly dependent on the Pt particle size. At room temperature, Pt particles stabilize ethylidyne as well as di-sigma- and pi-bonded ethylene; however, ethylidyne predominated on the surfaces of larger particles. Ethylidyne was the only identifiable species at 403 K, with its formation being more facile on larger particles. Co-adsorption experiments reveal that the composition of the surface layer is dependent on the order of exposure to gases. Exposure of a C2H4-covered Pt surface to CO resulted in an approximately 50% decrease in chemisorbed CO compared to a fresh Pt surface. The nu(CO) appeared at 2050 cm-1 on Pt/SBA-15 pretreated with C2H4 at room temperature. The di-sigma-bonded and pi-bonded species are the most susceptible to displacement from the surface by CO. The formation of ethylidyne appeared to be less sensitive to the presence of adsorbed carbon monoxide, especially on larger particles. Upon exposure of C2H4 to a CO-covered Pt surface, little irreversible uptake occurred due to nearly 100% site blocking. These results demonstrate that carbon monoxide competes directly with ethylene for surface sites, which will have direct implications on the poisoning of the heterogeneously catalyzed conversion of hydrocarbons.
通过漫反射红外光谱和化学吸附法，研究了一系列单分散粒径范围为1.7至7.1 nm的Pt/SBA - 15催化剂上一氧化碳和乙烯的吸附及其顺序吸附。气体吸附取决于Pt颗粒尺寸、温度和气体暴露顺序。室温下，CO在Pt/SBA - 15上的吸附产生了一个归属于C - O伸缩振动的光谱特征：ν(CO)= 2075 cm⁻¹（1.9 nm）；2079 cm⁻¹（2.9 nm）；2082 cm⁻¹（3.6 nm）；以及2090 cm⁻¹（7.1 nm）。信号强度随温度升高呈S形下降，从而提供了半定量的表面覆盖信息。乙烯在Pt/SBA - 15上的吸附产生了分别归属于亚乙基、双σ键合乙烯和π键合乙烯的约1340、约1420和约1500 cm⁻¹处的光谱特征。这些表面物种的比例高度依赖于Pt颗粒尺寸。在室温下，Pt颗粒稳定亚乙基以及双σ键合和π键合乙烯；然而，亚乙基在较大颗粒表面占主导。在403 K时，亚乙基是唯一可识别的物种，其在较大颗粒上的形成更容易。共吸附实验表明，表面层的组成取决于气体暴露顺序。与新鲜Pt表面相比，将覆盖有C₂H₄的Pt表面暴露于CO会导致化学吸附的CO减少约50%。在室温下用C₂H₄预处理的Pt/SBA - 15上，ν(CO)出现在2050 cm⁻¹处。双σ键合和π键合物种最容易被CO从表面置换。亚乙基的形成似乎对吸附的一氧化碳的存在不太敏感，尤其是在较大颗粒上。当将C₂H₄暴露于覆盖有CO的Pt表面时，由于几乎100%的位点被阻断，几乎没有发生不可逆的吸附。这些结果表明，一氧化碳与乙烯直接竞争表面位点，这将对烃类多相催化转化的中毒有直接影响。
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