15796
J. Phys. Chem. C 2008, 112, 15796–15801
Understanding How Surface Morphology and Hydrogen Dissolution Influence Ethylene Hydrogenation on Palladium Zdenek Dohna´lek,* Jooho Kim,‡ and Bruce D. Kay* Fundamental and Computational Sciences Directorate and Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352 ReceiVed: May 2, 2008; ReVised Manuscript ReceiVed: July 30, 2008
We prepared model supported nanoporous Pd catalysts grown via ballistic deposition with an unprecedented catalytic activity for ethylene hydrogenation. Extremely high conversion of ethylene to ethane (50%) exceeding that observed in prior studies by over an order-of-magnitude was achieved. Model Pd(111)/Pt(111) and Pddecorated Pd(111)/Pt(111) and FeO(111)/Pt(111) thin film catalysts were employed to examine how the reactivity evolves with creation of catalytically active Pd sites. The origin of enhanced reactivity is shown to be due to the presence of highly reactive Pd sites and the absence of surface hydrogen depletion due to dissolution into the bulk. Collectively these studies present an enhanced and unified understanding of the catalytic behavior of various Pd samples employed in prior studies. Introduction Palladium metal and its complexes have proven to be extremely useful as catalysts and are used in a broad range of applications, including catalytic converters in automobiles and mass production of various organic compounds produced in hydrogenation, dehydrogenation, and coupling reactions. Ethylene is a prototypical molecule used in catalytic hydrogenation studies, and as such, ethylene adsorption and reaction have been studied on a number of metals. On single-crystalline Pd(111),1-5 Pd(110),6-12 and Pd(100)13,14 surfaces the probability of making ethane in temperature-programmed desorption (TPD) experiments has been shown to be structure insensitive4,6 and occur with extremely low yield (0.001-0.01) under ultra-high-vacuum (UHV) conditions.4,10 Recent model studies on supported Pd particles showed a significant increase in the ethane yield per surface Pd atom which was attributed to the increased surface to bulk Pd ratio on the particles, thereby giving rise to reduced sorption of hydrogen into the bulk.4 The enhanced concentration of surface hydrogen is believed to result in the observed increase in catalytic activity. Nonetheless, even on these nanoparticles the C2H6 yield remains relatively low (0.01-0.04).4,15-17 Herein we demonstrate using temperature-programmed desorption experiments that C2H4 hydrogenation on nanoporous Pd films proceeds with remarkable initial C2H6 yield approaching 0.5 below room temperature. Repeated reaction cycles show that the extreme reactivity of the films also leads to catalyst poisoning via C2H4 dissociation. The Pd films are prepared via ballistic deposition (BD) at low temperatures (∼22 K) and high deposition angles (60-85°) using a directed Pd atom beam and possess extremely high surface areas where ∼25% of the atoms are available for reagent adsorption and reaction.18 Additionally the Pd film surface atoms have lower coordination than lowindex crystalline faces and hence exhibit enhanced reagent binding.18 Upon thermal annealing the films densify, resulting in dramatic reduction in catalytic activity. Ultimately, the films “smoothed” by annealing above 450 K exhibit low reactivity * Corresponding authors. E-mail:
[email protected], bruce.kay@ pnl.gov. ‡ Present address: LAM Research Corp., Fremont, CA 94538.
roughly equivalent to the supported nanoparticles. Model Pd(111)/Pt(111) and Pd-decorated Pd(111)/Pt(111) and FeO(111)/ Pt(111) thin film catalysts were employed to examine how the reactivity evolves with creation of catalytically active Pd sites. The origin of enhanced reactivity is shown to be due to the presence of highly reactive Pd sites and the absence of surface hydrogen depletion due to dissolution into the bulk. Collectively these studies present an enhanced and unified understanding of the catalytic behavior of various Pd samples employed in prior UHV surface science studies.1-17 Experimental Section The experiments were performed in a UHV apparatus with a base pressure of
