20020
J. Phys. Chem. C 2009, 113, 20020–20028
Reactivity of FeO(111)/Pt(111) with Alcohols Yu Kwon Kim,† Zhenrong Zhang, Gareth S. Parkinson, Shao-Chun Li, Bruce D. Kay,* and Zdenek Dohna´lek* Chemical and Materials Sciences DiVision, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, PO Box 999, Mail Stop K8-88, Richland, Washington 99352 ReceiVed: August 13, 2009; ReVised Manuscript ReceiVed: September 29, 2009
We report on the reactivity of a FeO(111) monolayer grown on Pt(111) toward aliphatic alcohols. Using a combination of temperature-programmed desorption, infrared reflection-absorption spectroscopy, and scanning tunneling microscopy techniques, we show that the alcohols dissociate primarily at FeO(111) step edges and their oxidation leads to the removal of the FeO(111) film. Upon annealing, FeO(111) lattice oxygen is incorporated into the reaction products, and reduced iron left behind dissolves into the underlying Pt(111) substrate. Ethanol is employed in a more detailed spectroscopic study to follow the reaction products and surface intermediates as the removal of FeO(111) proceeds. The ethoxy species formed upon dissociative adsorption of ethanol at the FeO(111) step edges undergo partial oxidation to acetaldehyde and a complete oxidation to CO and H2O. Other products, CH4 and H2, associated with the reactions occurring on Pt(111) are also observed as the bare Pt(111) surface appears. A similar etching process was also observed for n-decane. 1. Introduction Well-ordered thin films of oxides grown on metal substrates have proven extremely useful in model studies of catalytically and environmentally relevant reactions on oxides and oxide supported metal and metal oxide clusters.1-6 By employing such thin films, charging can be largely avoided, and therefore electron-based analytical techniques can be used for the investigations. A number of oxide thin films such as γ-Al2O3(111) on NiAl(110),2,7 MgO(100) on Mo(100) and Ag(100),8,9 SiO2 on Mo(112),10 TiOx on Mo(112) and Pt(111),11,12 FeO(111) on Pt(111),5,13-15 and Fe3O4(111) on Pt(111)5 have been prepared in this fashion. While the utility of these films has been proven in numerous studies,1-6 their properties do not always necessarily represent those of their bulk analogues. Our study of alcohols on FeO(111)/Pt(111) described here was motivated by our long-standing interest in understanding the catalytic activity of supported early transition metal oxide clusters.16-19 While FeO(111) is certainly not a typical catalytic support, our intent was to utilize its fully oxygen terminated surface5 as a nonreactive support for monodispersed (WO3)3 clusters.16,17 Our prior studies as well as studies from other groups have shown that the FeO(111) is unreactive toward number of adsorbates such as H2O20-22 and/or chlorinated hydrocarbons.15,23 In light of these studies, our expectation was that this surface would be unreactive toward alcohols as well. To date, only a few reports indicate that FeO(111) films can be unstable toward reduction and/or structural dewetting.23-27 Huang and Ranke24 have shown that atomic hydrogen dosed on FeO(111) can chemically reduce the film by removing surface oxygen via water formation, and Sun et al.27 have shown that under high-pressure reaction conditions a mixture of 40 mbar of CO and 20 mbar of O2 can cause dewetting and formation of highly dispersed FeOx nanoparticles. * To whom correspondence should be addressed: Ph (509) 371-6143, Fax (509) 371-6145, e-mail
[email protected] (B.D.K); Ph (509) 3716150, Fax (509) 371-6145, e-mail
[email protected] (Z.D.). † Current address: Division of Energy Systems Research, BK21 Program, Ajou University, Suwon 443-749, South Korea.
Here, we investigate reactions between primary aliphatic alcohols (methanol, ethanol, 1-butanol, and 1-octanol) and FeO(111) deposited on Pt(111). Our results indicate that the FeO(111) is not stable and can be completely removed by repeated cycles of alcohol adsorption at 80 K and annealing to 1000 K. In this process, lattice oxygen is utilized in the formation of oxidation products after alcohol dissociation. Iron that is left behind dissolves into the underlying Pt(111) upon high-temperature annealing. Scanning tunneling microscopy (STM) results show that the film etching is initiated from the step edges. On the basis of temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS) results, we propose a possible mechanism leading to etching of the FeO(111) film. 2. Experimental Details The experiments were performed in two ultrahigh-vacuum (UHV) systems: a molecular beam scattering apparatus, devoted to ensemble averaged studies, and a scanning probe microscopy apparatus, used for atomically resolved studies. The experimental setup and procedures used in both systems are described below. 2.1. Molecular Beam Scattering Apparatus. The molecular-beam scattering apparatus (base pressure
