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Sci Rep ; 6: 21990, 2016 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-26915303


Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO2(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO2.

J Chem Phys ; 138(2): 024704, 2013 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-23320710


The electronic structure of Cu(2)O and CuO thin films grown on Cu(110) was characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The various oxidation states, Cu(0), Cu(+), and Cu(2+), were unambiguously identified and characterized from their XPS and XAS spectra. We show that a clean and stoichiometric surface of CuO requires special environmental conditions to prevent loss of oxygen and contamination by background water. First-principles density functional theory XAS simulations of the oxygen K edge provide understanding of the core to valence transitions in Cu(+) and Cu(2+). A novel method to reference x-ray absorption energies based on the energies of isolated atoms is presented.

ACS Nano ; 6(12): 10743-9, 2012 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-23140267


The stability of Pt nanoparticles (NPs) supported on ultrathin SiO(2) films on Si(111) was investigated in situ under H(2) and O(2) (0.5 Torr) by high-pressure X-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 °C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtO(x) desorption from the Pt NPs (~2 and ~4 nm NP sizes) at temperatures above 450 °C in the presence of 0.5 Torr of O(2). Under oxidizing conditions, the entire NPs were oxidized, although with no indication of a PtO(2) phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e., minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment.

J Am Chem Soc ; 133(28): 10692-5, 2011 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-21707081


The reduction of a single-layer FeO film grown on Pt(111) by CO at elevated pressures and temperatures has been studied through an interplay of scanning tunneling microscopy, ambient-pressure X-ray photoelectron spectroscopy, and density functional theory calculations. Exposure of the FeO thin film to CO at pressures between 1 and 30 Torr and temperatures between 500 and 530 K leads to formation of a honeycomb-structured Fe(3)O(2) film with hollow sites occupied by single Pt atoms extracted from the substrate surface. The formation of these adatoms is driven by an increase in CO adsorption energy. In addition, the structure incorporates undercoordinated Fe centers, which are proposed to have substantial effects on the catalytic properties of the surface.

ACS Nano ; 5(7): 5987-94, 2011 Jul 26.
Artigo em Inglês | MEDLINE | ID: mdl-21671628


ZnO is a wide band gap metal oxide with a very interesting combination of semiconducting, transparent optical and catalytic properties. Recently, an amplified interest in ZnO has appeared due to the impressive progress made in nanofabrication of tailored ZnO nanostructures and functional surfaces. However, the fundamental principles governing the structure of even the clean low-index ZnO surfaces have not been adequately explained. From an interplay of high-resolution scanning probe microscopy (SPM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments, and density functional theory (DFT) calculations, we identify here a group of hitherto unresolved surface structures which stabilize the clean polar O-terminated ZnO(0001) surface. The found honeycomb structures are truly remarkable since their existence deviates from expectations using a conventional electrostatic model which applies to the opposite Zn-terminated (0001) surface. As a common principle, the differences for the clean polar ZnO surfaces are explained by a higher bonding flexibility of the exposed 3-fold coordinated surface Zn atoms as compared to O atoms.

J Am Chem Soc ; 132(9): 2858-9, 2010 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-20146473


The interaction of O(2) with gold foil and gold nanoparticles grown by thermal deposition on TiO(2)(110) was studied by in situ ambient pressure X-ray photoelectron spectroscopy at room temperature. No spontaneous dissociation of O(2) was observed either on Au foil or on Au nanoparticles up to 1 Torr of O(2). X-ray irradiation, however, is very effective in promoting gold oxidation on both surfaces in the presence of O(2). Our results help reconcile recent conflicting experimental observations regarding the activation of molecular oxygen, which is a crucial issue in Au catalyzed oxidation reactions.

Ouro/química , Oxigênio/química , Temperatura , Espectroscopia Fotoeletrônica