Atomically Dispersed Au-(OH) x Species Bound on Titania Catalyze the Low-Temperature Water-Gas Shift Reaction Ming Yang, † Lawrence F. Allard, ‡ and Maria Flytzani-Stephanopoulos* ,† † Department of Chemical and Biological Engineering, Tufts University, Medford, Massachusetts 02155, United States ‡ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: We report a new method for stabilizing appreciable loadings (∼1 wt %) of isolated gold atoms on titania and show that these catalyze the low-temperature water-gas shift reaction. The method combines a typical gold deposition/precipitation method with UV irradiation of the titania support suspended in ethanol. Dissociation of H 2 O on the thus-created Au-O-TiO x sites is facile. At higher gold loadings, nanoparticles are formed, but they were shown to add no further activity to the atomically bound gold on titania. Removal of this “excess” gold by sodium cyanide leaching leaves the activity intact and the atomically dispersed gold still bound on titania. The new materials may catalyze a number of other reactions that require oxidized active metal sites. A tomically dispersed supported metal catalysts offer new prospects for low-cost, sustainable energy and chemicals production, as discussed in a recent review. 1 The water-gas shift (WGS) reaction, which is important in upgrading H 2 -rich fuel gas streams for fuel cell and other applications, has been shown to occur on atomically dispersed metals on oxide supports. Thus, novel catalyst designs that maximize the number of such sites have been actively investigated. Along with improved Cu/ ZnO catalyst designs, 2 the Pt-group metals 3-7 are promising catalysts, as is gold when highly dispersed on a support such as ceria 8-15 or iron oxide. 12,15-18 Gold supported on titania has not been examined extensively for the WGS reaction. Of course, this was one of the first catalysts reported by Haruta and co-workers as an extremely active catalyst for ambient- temperature oxidation of carbon monoxide. 19,20 Au/TiO 2 prepared by a deposition/precipitation (DP) technique was recently investigated for the WGS reaction. 21,22 On the basis of kinetic data and geometric arguments, it was proposed that the corner atoms on the gold cuboctahedral nanoparticles with fewer than seven neighboring gold atoms are the dominant active sites and that the total rate is proportional to the number of gold particles but does not depend on their size. Gold nanoclusters and isolated gold atoms on titania were not included in the counting of active gold species in that work. 22 However, recent atomic-resolution imaging studies via aberration-corrected scanning transmission electron micros- copy (ac-STEM) clearly showed that even a minor amount of atomically dispersed gold on the titania surface benefits the CO oxidation reaction 23 as well as various dehydrogenation reactions. 24 These gold species are also very active for the WGS reaction on ceria and doped ceria, 8-15,25 iron oxide, 12,15-18 zirconia, 26,27 and lanthana. 28 Indeed, for WGS- active gold supported on the above oxides, gold nanoparticles can be leached out by alkali cyanide solutions, and the residual gold (a small fraction of the original amount) on these supports was found to catalyze the reaction equally well. 8-10,12,28 Cyanide leaching allows for isolated gold species to be imaged and their reactivities be followed by various techniques in the absence of particles that would distort the data (e.g., in IR, XANES, XPS, and other “averaging” techniques). For the Au/ Fe 2 O 3 WGS catalysts, Allard and co-workers 17,18 showed that atomic gold species are strongly bound even after redox heat treatments and after exposure to the WGS reaction gas mixture up to 673 K. In work with Au/Fe 3 O 4 (111) single crystals, gold atoms bound over the uncapped O atoms were shown to be stable to 773 K. 29,30 Thus, renewed effort should be spent on properly characterizing the atomically dispersed Au-O x oxide sites, even though they are not visible by regular TEM. Qiao et al. 31 showed that a single Pt atom place-exchanged in an FeO x surface has excellent CO oxidation activity in the preferential CO oxidation reaction. As reported by Fu and co-workers, 8,10 the sodium cyanide leaching of the gold must be done only after oxidative heat treatment of the fresh material when the DP technique is used to add the gold. This allows for strong Au-O(OH) x -Ce association to take place first, which resists cyanide complex- ation. Otherwise, almost all of the deposited gold is leached out even from the ceria surfaces. To date, it has been challenging to retain the atomically dispersed Au-O x species on TiO 2 . For titania supports, a different energy stimulation must be used to create stable anchoring sites for gold. For example, using a model Au/TiO 2 film, Lahiri et al. 32 showed that UV irradiation enabled the stabilization of the semiconductor-metal interface through the accumulation of gold cations, presumably at surface defect sites. Recent computational work by Laursen and Linic 33,34 has shown that oxygen vacancies are not needed to stabilize cationic gold on titania, and the electron-rich defects are energetically unfavorable and would be healed rapidly in H 2 O or O 2 environments. This was corroborated by scanning tunneling microscopy studies, 35 which revealed that the image features earlier attributed to oxygen vacancies on well-annealed TiO 2 single crystals are actually protons atop bridging oxygen or -OH groups bound to Ti through atoms. In the present work, we have extended the UV irradiation technique to titania Received: January 3, 2013 Published: February 25, 2013 Communication pubs.acs.org/JACS © 2013 American Chemical Society 3768 dx.doi.org/10.1021/ja312646d | J. Am. Chem. Soc. 2013, 135, 3768-3771