J. Phys. Chem. C 2010, 114, 2293–2299
2293
Role of Hydrogen Species in Palladium-Catalyzed Alkyne Hydrogenation Detre Teschner,*,†,‡ Ja´nos Borsodi,†,‡ Zolta´n Kis,‡ La´szlo´ Szentmiklo´si,‡ Zsolt Re´vay,‡ Axel Knop-Gericke,† Robert Schlo¨gl,† Daniel Torres,§ and Philippe Sautet§ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, Berlin, D-14195, Germany, Institute of Isotopes, Hungarian Academy of Science, POB 77, Budapest, H-1525, Hungary, and UniVersite´ de Lyon, CNRS, Institut de Chimie, Laboratoire de Chimie, Ecole Normale Supe´rieure de Lyon, 46 Alle´e d’Italie, 69364 Lyon, Cedex 07, France ReceiVed: October 30, 2009; ReVised Manuscript ReceiVed: December 10, 2009
Selective alkyne hydrogenation in the presence of carbon-carbon double bond compounds, for which Pd is an excellent catalyst, is a strategically important large-scale industrial process. Although in palladium, functionality and structure are closely interrelated, knowledge of the structure of Pd is insufficient as the interaction with the chemical environment causes drastic compositional changes near the subsurface region: while unselective hydrogenation proceeds in the presence of a β-hydride phase, selective hydrogenation can be achieved only in the presence of a near-surface Pd-C phase. Here, we show from a combination of in situ prompt gamma activation analysis and ab initio simulations based on density functional theory that (i) the presence of the Pd-C phase created under selective conditions implies a strong change in the surface and subsurface stability of hydrogen, (ii) there is still a slower exchange of bulk and surface hydrogen, and (iii) the reaction rate for selective hydrogenation is independent of the bulk H/Pd ratio. 1. Introduction Removal of traces of alkynes or multiple unsaturated hydrocarbons from ethylene or propylene feed to produce polymergrade alkenes is achieved by selective hydrogenation. The aim in this process is to produce selectively the partial hydrogenation product and not to convert the alkene. Although few studies1,2 have recently reported that gold catalysts possess high selectivity in this reaction, the element of choice is still palladium. This metal is particularly interesting for hydrogenation because of the high solubility of hydrogen in the metal. However, this makes it difficult to obtain a full understanding of the properties of hydrogen on its surfaces. For example, it is a nontrivial matter to quantify the amount of adsorbed hydrogen present during the catalytic run because of the possibility of subsurface absorption and the very low sensitivity of the spectroscopic methods toward hydrogen. Selective hydrogenation of acetylene (and to a lesser extent propyne) is a well-studied reaction, and the genesis of the active surface requires a strong modification of the near-surface region of Pd.3 The significant fragmentation of reactant molecules leads to the formation of an active Pd-C surface phase, composed of alternating Pd and C atomic layers on top of metallic Pd.4 Moreover, it is typically observed that the reaction is close to first-order with respect to hydrogen and zero- or negative-order for the alkyne.5–11 The conclusion drawn usually from such power-law parameters is that hydrogen in the presence of alkynes adsorbs only weakly on palladium. Its surface coverage should be accordingly low (and the hydrogen concentration in the bulk is even lower, assuming that hydrogen on the surface equilibrates with the bulk). This is in agreement with the simulated surface coverage of hydrogen, being in the range of * Corresponding author. Phone: +49-30-8413-5408. Fax: +49-30-84134676. E-mail:
[email protected]. † Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡ Hungarian Academy of Science. § Universite´ de Lyon.
0.06-0.1, as opposed to that of acetylene (0.33-0.31) during the acetylene hydrogenation event on Pd(111).12 However, at high hydrogen excess of the acetylene feed, Borodzinski13,14 observed the appearance of a hydride phase in Pd/Al2O3, with a corresponding increase in the activity and decrease in selectivity. Since an XRD camera was applied to follow the hydride formation, the concentration of hydrogen itself in palladium was not known. The hydrogen-palladium system has been the subject of many studies, both experimentally and computationally.15–17 From previous studies using low energy electron diffraction, it is known that on Pd(111), hydrogen forms ordered structures with (3 × 3)R30 periodicity, for hydrogen coverage lower than 1 ML and a (1 × 1) structure at 1 ML.16,18 A complex phase diagram characterizes the PdHx system. Two solid solutions are known: nominally, the R and β phases. The best characterized crystalline phase is β-PdHx (0.6 < x
