Catalytic reactivity of face centered cubic PdZn_伪 for the steam reforming of methanol

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• 作者：Barr Halevi^a ; ^{halevi@unm.edu} ; Eric J. Peterson^a ; Aaron Roy^a ; Andrew DeLariva^a ; Ese Jeroro^b ; Feng Gao^c ; Yong Wang^c ; ^d ; John M. Vohs^b ; Boris Kiefer^e ; Edward Kunkes^f ; Michael Hä ; vecker^f ; Malte Behrens^f ; Robert Schlö ; gl^f ; Abhaya K. Datye^a ; ^{datye@unm.edu}
• 关键词：PdZn ; Methanol steam reforming ; MSR ; Heterogeneous catalysis ; Intermetallic ; Spray pyrolysis ; Model catalyst ; Inter-metallic ; Aerosol
• 刊名：Journal of Catalysis
• 出版年：2012
• 期刊代码：97_00219517
• 类别：ce
• 出版时间：July, 2012
• 卷：291
• 期：Complete
• 页码：44-54
• 文件大小：1135 K

摘要

Addition of Zn to Pd changes its catalytic behavior for steam reforming of methanol. Previous work shows that improved catalytic behavior (high selectivity to CO₂) is achieved by the intermetallic, tetragonal L1₀ phase PdZn_尾1, where the Pd:Zn ratio is near 1:1. The Pd-Zn phase diagram shows a number of other phases, but their steady-state reactivity has not been determined due to the difficulty of precisely controlling composition and phase in supported catalysts. Hence, the role of Zn on Pd has generally been studied only on model single crystals where Zn was deposited on Pd(1 1 1) with techniques such as TPD and TPR of methanol or CO. The role of small amounts of Zn on the steady-state reactivity of Pd-Zn remains unknown. Therefore, in this work, we have synthesized unsupported powders of phase pure PdZn_伪, a solid solution of Zn in fcc Pd, using a spray pyrolysis technique. The surface composition and chemical state were studied using Ambient Pressure-XPS (AP-XPS) and were found to match the bulk composition and remain so during methanol steam reforming (MSR) (P_tot = 0.25 mbar). Unlike the PdZn_尾11 phase, we find that PdZn_伪 is 100%selective to CO during methanol steam reforming with TOF at 250 掳C of 0.12 s^鈭?. Steady-state ambient pressure micro-reactor experiments and vacuum TPD of methanol and CO show that the 伪 phase behaves much like Pd, but Zn addition to Pd improves TOF since it weakens the Pd-CO bond, eliminating the poisoning of Pd by CO during MSR over Pd. The measured selectivity for fcc PdZn_伪 therefore confirms that adding small amounts of Zn to Pd is not enough to modify the selectivity during MSR and that the PdZn_尾1 tetragonal structure is essential for CO₂ formation during MSR.