Individual Heat of Adsorption of Adsorbed CO Species on Palladium and Pd–Sn Nanoparticles Supported on Al2O3 by Using Temperature-Programmed Adsorption Equilibrium Methods

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• 作者：Imen Jbir ; Julien Couble ; Sihem Khaddar-Zine ; Zouhaier Ksibi ; Fréderic Meunier ; Daniel Bianchi
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：April 1, 2016
• 年：2016
• 卷：6
• 期：4
• 页码：2545-2558
• 全文大小：639K
• ISSN：2155-5435

文摘

The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al₂O₃ and 2% Pd-x% Sn/Al₂O₃ (weight %, x = 2 or 5 wt %) in the 300–713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using Fourier transform infrared (FTIR) spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted as “B”), which are the dominant species on Pd⁰ particles and (b) a significant increase in the Pd sites forming linear CO species (denoted as “L”). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature-programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al₂O₃, in parallel to the formation of two strongly adsorbed B CO species, two linear L1_Pd⁰ and L2_Pd⁰ CO species are formed, which exhibit different heats of adsorption. For the dominant L1_Pd⁰ CO species, the heat of adsorption decreases linearly, from 92 kJ/mol to 54 kJ/mol, as its coverage increases at coverage 0 and 1, respectively, while that of the L2_Pd⁰ species is >165 kJ/mol at coverage 1. On the two Pd–Sn containing particles, two linear CO species are formed denoted L1_2Pd–xSn and L2_2Pd–xSn with x = 2 or 5. The L1_2Pd–xSn species dominates the CO adsorption on the two solids. It is shown that its heat of adsorption (which is slightly dependent on x) linearly varies with its coverage: ∼90 kJ/mol and ∼50 kJ/mol at low and high coverage, respectively. The comparison with the heat of adsorption of L1_Pd⁰ indicates that the electronic effect of tin is very modest, compared to its geometric effect. This conclusion is consistent with literature data dedicated to DFT calculation. Moreover, (a) XRD and TEM/EDX analysis suggest that bimetallic particles such as Pd₃Sn and Pd₂Sn are present and (b) the impact of tin on the H₂ chemisorption on Pd⁰ sites are presented.