Adsorption of carbon monoxide Au/Pd(1 0 0) alloys in ultrahigh vacuum: Identification of adsorption sites Zhenjun Li a , Feng Gao b , Octavio Furlong a , Wilfred T. Tysoe a, * a Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA b Department of Chemistry, Texas A&M University, College Station, TX 77840, USA article info Article history: Received 28 August 2009 Accepted for publication 28 October 2009 Available online 31 October 2009 Keywords: Reflection–adsorption infrared spectroscopy Temperature-programmed desorption X-ray photoelectron spectroscopy Low-energy ion scattering Chemisorption Palladium–gold alloy Carbon monoxide abstract The adsorption of carbon monoxide is studied on Au/Pd(1 0 0) alloys by means of reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The alloy was formed by adsorbing a four-monolayer thick gold film on a Pd(1 0 0) substrate and by heating to various tempera- tures to form alloys with a range of palladium coverages. The alloy was characterized using X-ray pho- toelectron spectroscopy and the composition of the outermost layer measured using low-energy ion scattering spectroscopy. CO adsorbs on palladium bridge sites only for palladium coverages greater than 0.5 monolayers (ML) suggesting that next-nearest neighbor sites are preferentially populated by palla- dium atoms. CO adsorbs on atop palladium sites and desorbs at 350 K corresponding to a desorption activation energy of 117 kJ/mol. However, at lower palladium coverages, these sites are not occupied and CO desorption states are detected 170 and 112 K corresponding to desorption activation energies of 53 kJ/mol and 35 kJ/mol, respectively, for these states. It is suggested that these states are due to a restructuring of the surface to form low-coordination gold sites that obscure the atop palladium site. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Alloys have been widely used as heterogeneous catalysts [1,2] by adding a second metal to improve activity and/or selectivity. Palladium–gold alloys have been found to be effective for a number of reactions such as CO oxidation, cyclotrimerization of acetylene to benzene, vinyl acetate synthesis, selective oxidation of alcohols to aldehydes or ketones, oxidation of hydrogen to hydrogen perox- ide or water, and hydrocarbon hydrogenation [3–19]. The nature of the activation/selectivity promotion, therefore, has attracted much research interest. In particular, the (1 0 0) face of gold–palladium alloy single crystals has been found to particularly effective for vi- nyl acetate synthesis and this effect has been rationalized by an ensemble effect [6]. Scanning-tunneling microscopy (STM) of the surface of an AuPd(1 0 0) alloy single crystal has suggested that ensembles consisting of two palladium atoms located at opposite corners of the (1 0 0) surface unit cell occur with a greater fre- quency than would be expected for a random distribution [20]. Non-random distributions of gold and palladium in Au/Pd(1 1 1) alloys have also been observed [21,22]. Carbon monoxide is a potentially useful probe for the surface structures of alloys since the desorption temperatures and, in particular, the CO stretching frequencies are very sensitive to the nature of the adsorption site [23–32]. In addition, since these gold–palladium alloys are potentially active CO oxidation catalysts [33,34], an investigation of CO on Au/Pd(1 0 0) alloys may provide insights into this chemistry. The utility of carbon monoxide as a probe for adsorption sites on gold–palladium alloys has been illustrated for Au/Pd(1 1 1) al- loys, where desorption features were detected at 255, 334, 383 and 451 K where the 255-K feature is assigned to CO adsorbed on two adjacent atop sites, where one CO desorbs at low tempera- tures allowing the second CO to move to a more stable site. The 334, 383 and 451 K features are due to desorption from atop, bridge and threefold hollow sites, respectively [32,35]. Insights into the dependence of heat of adsorption (i.e. desorption activa- tion energy) on the nature of the adsorption site and the effects of lateral interactions between sites has come from density func- tional theory (DFT), which reveals that the adsorption site has the largest influence on the heat of adsorption, while lateral inter- actions are less important but still have a significant effect [35]. Two CO desorption peaks are found at 350 and 490 K for Pd(1 0 0), however RAIRS reveals that CO only occupies bridge sites on this surface up to a coverage of 0.5 ML, where it displays a c(2 p 2  p 2)R45° low-energy electron diffraction (LEED) pattern [25,36,37]. CO stretching frequencies between 1895 and 1950 cm 1 are found as a function of coverage below 0.5 ML in ac- cord with the assignment to adsorption in bridge sites. A further increase in coverage causes additional shifts in the vibrational fre- quency to 1995 cm 1 . 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.10.031 * Corresponding author. Tel.: +1 414 229 5222; fax: +1 414 229 5036. E-mail address: wtt@uwm.edu (W.T. Tysoe). Surface Science 604 (2010) 136–143 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc