Journal of Catalysis 203, 292–306 (2001) doi:10.1006/jcat.2001.3347, available online at http://www.idealibrary.com on The Influence of Catalyst Restructuring on the Selective Hydrogenation of Acetylene to Ethylene Yaming Jin, *,1 Abhaya K. Datye, *,2 Ed Rightor,† Robert Gulotty,† Wendy Waterman,† Michael Smith,† Michael Holbrook,§ Joe Maj,† and John Blackson† * Center for Microengineered Materials and Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131; †Dow Chemical Company, Midland, Michigan 48667; ‡Dow Chemical Company, Freeport, Texas 77541; and §Dow Chemical Company, Plaquemine, Louisiana 70765 Received December 26, 2000; revised June 18, 2001; accepted June 28, 2001 Structural changes were induced in Pd and Pd–Ag catalysts by oxidation and reduction. The influence of oxidative restructuring on the selectivity for acetylene hydrogenation to ethylene was studied under conditions of industrial significance (acetylene conversion >99%). We found that oxidative restructuring of the Pd-only catalyst had no influence on hydrogenation selectivity; neither was any correlation found between selectivity and extent of β -Pd hydride formation. When Ag was added as a promoter, however, oxidation–reduction treatments had a major impact on selectivity. High-resolution transmission electron microscopy showed rough surfaces were created by oxidation of the catalyst which were coincident with poor selectivity. Annealing the catalyst at progressively higher temperatures in H 2 improved the selectivity to ethylene. Reduction in H 2 at 500 ◦ C also suppressed the formation of oligomeric hydrocarbon by-products during reaction. In situ infrared study of CO adsorption showed that the addition of Ag to the Pd/silica results in an ensemble or geometric effect, increasing the proportion of linear CO relative to the bridged form. We infer that oxidation causes the Pd and Ag to segregate within the particle. High-temperature annealing in H 2 causes a migration of the promoter Ag so as to yield improved selectivity on these catalysts. The silica support surface also seems to provide a suitable environment to allow restructuring of the Pd–Ag crystallites to occur. c  2001 Academic Press Key Words: selective hydrogenation of acetylene in ethylene; Pd– Ag bimetallic catalysts, characterization; Pd–Ag, oxidation and re- duction of; IR spectroscopy of CO, on Pd–Ag. INTRODUCTION Small amounts of acetylene (<3%) are present in ethy- lene derived from thermal pyrolysis or steam cracking. This acetylene is a poison for Ziegler–Natta polymerization cata- lysts and must be removed. Palladium-based catalysts have been used to selectively hydrogenate the acetylene impurity 1 Present address: Conoco Inc., Ponca City, OK 74602-1267. 2 To whom correspondence should be addressed. E-mail: datye@ unm.edu. in the ethylene feedstock (1). The catalyst must meet two important criteria: selective hydrogenation of acetylene in the presence of a large amount of ethylene, and catalyst lifetime, which is governed by the deposition of oligomeric by-products (called green oil) on the catalyst. Hydrocarbon deposition fouls the catalyst, requiring regeneration by an oxidative treatment. The catalyst then needs to be reduced in H 2 before use, or simply activated in situ. Catalyst se- lectivity changes with time on stream, presumably due to changes in structure, as well as to the deposition of hydro- carbon species on the metal surface. Our work is directed at understanding the restructuring of the Pd catalysts as they are activated for reaction by reduction. Selective hydrogenation of trace acetylene over Pd-based catalysts has been extensively studied. Some of the factors that have been suggested to alter catalyst selectivity include Pd particle size (2, 3), the presence of adatoms (4), and the formation of palladium hydride (2, 3). The interaction of these variables on the role of restructuring has not been explicitly investigated. Silver and other Group IB metals are commonly used additives for Pd catalysts used for se- lective hydrogenation of acetylene [e.g., (4–6)] and also of diolefins (7). Although widely used, the nature of the Pd– Ag bimetallic particles and the exact mechanism by which Ag modifies the selectivity of Pd catalysts are poorly under- stood. In industrial operation, the acceptable acetylene content in the effluent should be lower than 10 ppm, and prefer- ably less than 5 ppm (1), which corresponds to an acetylene conversion higher than 99%. The temperature where this is achieved is called the cleanup temperature. As reactor temperature is further increased, runaway ethylene hydro- genation can occur, which is manifested in an increase in the ethane/ethylene molar ratio. The temperature range be- tween cleanup and runaway is of practical importance from an operational standpoint, since it is desirable to remove all of the acetylene from the ethylene stream without excessive loss of ethylene. Unfortunately, most academic research is 0021-9517/01 $35.00 Copyright c  2001 by Academic Press All rights of reproduction in any form reserved. 292