Electrochemical Promotion by Potassium of the Selective Hydrogenation of Acetylene on Platinum: Reaction Studies and XP Spectroscopy Federico J. Williams, Alejandra Palermo, Samuel Tracey, Mintcho S. Tikhov, and Richard M. Lambert* Department of Chemistry, Cambridge UniVersity, Lensfield Road CB2 1EW, England ReceiVed: February 11, 2002; In Final Form: March 20, 2002 In the partial hydrogenation of acetylene, potassium, pumped to a thin film Pt catalyst from the electro-active support K ′′-alumina, very strongly promotes ethylene formation. In the best case, K-promotion increases ethylene selectivity from <20% (for the un-promoted catalyst) to 90%. XPS and photoelectron microscopy show that under these conditions submonolayer amounts of K are uniformly distributed on the Pt surface and that alkali transport, like the catalytic behavior, is fully reversible as a function of catalyst potential. Auxiliary experiments strongly suggest that ethylene, or a surface species derived from it, is a key intermediate in the total hydrogenation of acetylene to ethane. Promotion of selective hydrogenation by potassium may be rationalized in terms of K-induced weakening and strengthening respectively of ethylene and hydrogen adsorption. Comparison with earlier results indicates that the associated electronic effects are relatively insensitive to the chemical identity of the alkali. Introduction The palladium-catalyzed removal of acetylene impurity from ethylene feed streams 1-3 is technically important, although despite intensive technical development the reaction mechanism is still not well understood. 4 In comparison to palladium, platinum is a very poor selective hydrogenation catalyst, 5 producing mainly ethane under most conditions, probably because of its overly strong interaction with adsorbed acetylene. 6-9 However, we may deliberately exploit this fact in order to shed light on the reaction mechanism by using promoted Pt catalysts to study the partial hydrogenation of acetylene. In the case of platinum catalysts, one would expect promoter effects to be relatively large and certainly much greater than in the case of Pd. Proceeding thus, we may hope to elucidate promoter action and, hence, reaction mechanism. Recently, using the technique of electrochemical promotion (EP), we showed that when sodium was supplied to a thin Pt film catalyst from a solid electrolyte support the selectivity for partial hydrogenation of acetylene to ethylene was greatly enhanced. 10 Here, we report on extension of this work in several important ways. First, we investigated the effects of potassium promotion in order to examine whether chemically specific effects are significant. Second, spectroscopic measurements were performed in order to establish that EP by K of a Pt film does indeed involve reversible supply of the potassium to the surface of the platinum catalyst; these data were previously unavailable. Finally, auxiliary experiments with ethylene were carried out to examine certain aspects of the reaction mechanism. The EP technique, discovered and developed by Vayenas and his school, 11 entails electrochemical pumping of ions from a solid electrolyte to the surface of a porous, catalytically active metal film with which it is in contact. A recent concise review of the phenomenology, methodology, and underlying theory of electrochemical promotion by alkalis is available. 12 In the present case, the solid electrolyte was K- ′′ alumina (a K + conductor). Under forward bias (catalyst working electrode negative relative to counter electrode), K + ions are transported to the catalyst electrode where they are discharged at the metal electrode/solid electrolyte/gas three phase boundary. The result- ing species (K) are thought to spill over onto the surface of the metal catalyst, thus altering its reactive behavior. For example, EP by Na of the Pt-catalyzed NO+CO and NO+ propene reactions 13,14 induces platinum to behave like rhodium. As we shall see, in the case of acetylene selective hydrogenation, EP by alkali causes platinum to behave like palladium. Experimental Methods The method of sample preparation and cleaning, the reactor geometry, analysis system, and electrochemical setup have already been described in detail. 10 The overall methodology closely follows that established by Vayenas and co-workers. 15 Briefly, the platinum catalyst (working electrode, W) consisted of a porous continuous thin film (∼1 cm 2 geometric area) deposited by DC sputtering on one face of a 10 mm × 15 mm K- ′′-Al 2 O 3 wafer. Au reference (R) and counter (C) electrodes were deposited on the other face of the solid electrolyte wafer, also by sputtering. Electrical contact with the W, R, and C electrodes was by means of 0.5 mm diameter Au wires, which also served to suspend the sample in a quartz vessel (35 cm 3 ), and the overall system behaved as a single pellet stirred tank reactor. 16 The mode of operation is as follows. Imagine starting with the Pt catalyst film covered by some K. Under potentiostatic conditions, imposition of a positive potential (V WR > 0) between working electrode (i.e., the Pt catalyst film) and reference electrode results in current flow between the working electrode (catalyst) and Au counter electrode corresponding to the following anodic reaction at the Pt working electrode: This occurs until the amount of K pumped away from the Pt causes the value of V WR to reach the desired pre-set value, at * To whom correspondence should be addressed. E-mail: RML1@ cam.ac.uk. Fax: +44 1223 33 6362. Phone: +44 1223 33 6467. K(Pt) f K + (′′-Al 2 O 3 ) + e - (1) 5668 J. Phys. Chem. B 2002, 106, 5668-5672 10.1021/jp0203954 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002