Catalysis Letters Vol. 71, No. 1-2, 2001 1 On the effect of hydrogen on the palladium-catalyzed formation of benzene from acetylene D. Stacchiola, G. Wu, H. Moleroand W.T. Tysoe ∗ Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA E-mail: wtt@uwm.edu Received 8 September 2000; accepted 3 November 2000 An infrared spectrum of a Pd(111) surface collected in the presence of 5 Torr of acetylene as a function of hydrogen pressure reveals that the ethylidyne coverage increases with hydrogen pressure (P (H 2 ) between zero and 20 Torr). The amount of CO that can be accommodated onto the surface at a pressure of 5 Torr, measured after evacuating the acetylene and hydrogen, increases linearly with hydrogen pressure, and this effect is ascribed to the presence of a more open surface produced by the formation of ethylidyne. It is found that acetylene adsorbs in ultrahigh vacuum on ethylidyne-covered Pd(111) and reacts to form benzene, where the benzene desorbs at ∼280 K. This effect is mirrored in the catalytic chemistry where the rate of benzene formation from acetylene in the presence of hydrogen increases linearly with hydrogen pressure. KEY WORDS: acetylene cyclotrimerization; Pd(111) model catalyst; effect of hydrogen; reflection–absorption infrared spectroscopy; temperature-programmed desorption spectroscopy; catalytic reactions 1. Introduction Palladium-catalyzed formation of benzene from acety- lene has been extensively studied over the past twenty years [1,2]. This is a particularly attractive reaction for fundamen- tal studies since it proceeds both under ultrahigh vacuum conditions [3] as well as at high pressures [4,5]. Work on probing the reaction in ultrahigh vacuum on clean Pd(111) has shown that benzene is rapidly formed from acetylene following adsorption at ∼100 K via reaction of a C 4 H 4 met- allacyclic intermediate with adsorbed C 2 H 2 [6], and desorbs in two states at ∼280 and 520 K with a small peak detected at ∼380 K. These are desorption-rate-limited states, where the low-temperature (∼280 K) feature is assigned to the desorp- tion of tilted benzene desorbing from a crowded surface, and the 380 and 520 K states to the desorption of flat-lying ben- zene [7]. The surface “crowding” can also be simulated by co-adsorbing acetylene with NO [8]. More recently the pres- ence of the tilted and flat-lying species has been confirmed using high-resolution X-ray photoelectron spectroscopy [9]. Under catalytic conditions, however, the Pd(111) surface is covered by a vinylidene monolayer ( vinylidene = 1.0 [3]). It has been shown that CO can still access the palladium sur- face, in spite of the presence of this crowded carbonaceous layer, where the CO saturates at a coverage of ∼0.15 mono- layers at a pressure of 5 Torr [10]. It has been demonstrated that the presence of the vinylidene overlayer modifies the surface reaction pathway so that benzene is formed under catalytic conditions via an initial reaction between acetylene adsorbed at high pressures onto the palladium surface, and vinylidene species [11], indicating that vinylidenes are not ∗ To whom correspondence should be addressed. inert spectator species. It has also been found that, both on high-surface-area, supported palladium catalysts [12], and on planar model systems [13], the addition of hydrogen to the acetylene, in addition to forming ethylene, enhances the rate of benzene formation. It has also recently been shown that an adsorbed vinylidene overlayer converts into ethyli- dyne species in the presence of high pressures (∼0.5 Torr) of hydrogen [14]. Furthermore, CO adsorbs much more eas- ily on ethylidyne-covered Pd(111) than on the vinylidene- covered surface [15]; in the latter case, CO pressures of ∼5 Torr are required to saturate the surface with CO at 300 K, while in the former case, three-fold hollow sites are occupied when exposed to 2 L of CO, and atop sites are all occupied at a CO pressure of ∼10 -2 Torr [15]. This sug- gests that a possible reason for the enhanced rate of benzene formation when the surface is exposed to a C 2 H 2 + H 2 mix- ture is that hydrogen reacts with adsorbed vinylidene species forming a more open ethylidyne-covered surface. This sug- gestion is tested in the following by monitoring the nature of the catalytic surface during reaction using infrared spec- troscopy. The openness of the surface is probed, after reac- tions on a Pd(111) single crystal, using CO chemisorption. This is compared to the rate of benzene formation where a good correlation is found between the two measurements. 2. Experimental Several pieces of apparatus were used for these ex- periments which have been described in detail elsewhere [10,16,17]. The first is a high-pressure catalytic reactor en- closed in an ultrahigh-vacuum chamber [16]. The reactor is a coaxial, high-pressure cell which can be sealed and 1011-372X/01/0100-0001$19.50/0  2001 Plenum Publishing Corporation