NANOSTRUCTURED OXIDES: NEW MATERIALS FOR ENERGY AND ENVIRONMENT

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1 NANOSTRUCTURED OXIDES: NEW MATERIALS FOR ENERGY AND ENVIRONMENT Quantum Chemistry Laboratory Dipartimento di Scienza dei Materiali Università Milano-Bicocca

2 THE GROUP AT UNIMIB

3 Quantum Chemistry Laboratory THREE MAIN RESEARCH LINES Nanostructured oxides and thin films as model catalysts (UHV, surface science) Semiconducting oxides in photocatalysis Band gap engineering Models of real catalysts Concepts and interpretations

4 Nanostructured oxides and thin films Model catalysts

5 Diffusion from gas phase N 2 NO Product desorption Dissociative CO Surface adsorption CO diffusion 2 Molecular adsorption Noble metal reaction Adsorbed product Support Real systems: Au nanocatalyst on TiO 2 TEM Model systems: Au nanoclusters on MgO/Ag(100) STM

6 METHODS: DENSITY FUNCTIONAL THEORY Density functional theory ( ) VASP (G. Kresse, J. Hafner (Technical Unv. of Vienna) vacuum Plane wave basis set Pseudopotentials + Projector Augmented Wave (PAW, Blöchl) DFT or DFT+U approach PW91, PBE, HSE functionals

7 GOLD NANOPARTICLES ON MgO ULTRATHIN FILMS G. Pacchioni, 26/3/2015 Electron transfer from oxide films to adsorbates (metal clusters, molecules) by electron tunneling Ricci, Bongiorno, GP, Landman, Phys. Rev. Lett. 97, (2006) Sterrer, Risse, Heyde, Rust, Freund, Phys. Rev. Lett. 98, (2007)

8 STRUCTURAL CHANGE OF FeO/Pt(111) FILM UNDER O 2 PRESSURE Giordano, Goniakowski, Shaikutdinov, GP, Freund et al. Angew. Chemie 49, 4418 (2010)

9 MAIN RESULTS on ultrathin films spontaneous charging of supported clusters possible by direct tunneling; role of the metal/oxide work function structural flexibility may lead to enhanced reactivity Exploit nano-porosity to modify properties and generate assembly of ad-atoms and clusters Doping of oxide films - charging can be induced by selective doping with TM atoms

10 Semiconducting oxides in photocatalysis Band gap engineering

11 Photoactivity: the mechanism R Abe, J.Photochem.Photobio.C. 2011, 11, 179. A photon of suitable frequency excites an electron (e - ) from VB to CB generating a hole (h + ) in the valence band Recombination processes reduce efficiency Photon frequency needed for excitation depends on band gap value. The redox potential depends on the energy of e - (CB) and h + (VB) with respect to the potential of the semi-reaction involved

12 METHODS: DENSITY FUNCTIONAL THEORY ( ) Dovesi, Saunders, Roetti, Orlando, Zicovich- Wilson, Pascale, Civalleri, Doll, Harrison, Bush, D Arco, Llunell, CRYSTAL09 (University of Torino) band structure theory periodic supercell Atomic orbital basis set Allows one to treat exchange exactly (HF) and to use hybrid DFT functionals (B3LYP) SEMICONDUCTING OXIDES: BAND GAP PROBLEM TiO 2 ZnO ZrO 2 WO 3

13 DOPED TiO 2 : B, C, N, F SUBSTITUTIONAL TO O Paramagnetic defects states in the gap move down along the series F substitutional: formation of Ti 3+ (3d 1 ) States are localized! Systematic DFT-B3LYP calculations Di Valentin, GP, Catal. Today 206, 12 (2013)

14 DOPED TiO 2 : B, C, N INTERSTITIALS Interstitials more stable than substitutional B and C interstitials act as electron donors: formation of Ti 3+ (3d 1 ) N forms NO interstitials (proven by EPR and XPS) Defect states are localized!

15 Third generation photocatalysts Multi-photon excitations? ZrO 2, inert material as photocatalyst due to high band gap Ce-doped ZrO 2 : under visible light material becomes active e /h + pairs generated by irradiation with visible light ( > 420 nm) e /h + pairs migrate to the surface Ce introduces new states in the gap Double excitation possible explanation Gionco, Paganini, Giamello, Burgess, Di Valentin, GP, J. Phys. Chem. Lett. 5, 447 (2014)

16 SEMICONDUCTING OXIDES: DEFECTS, DOPANTS, NANOSTRUCTURES Oxygen vacancy in WO 3 : electrochromic windows Charge state of O vacancy may explain effect; strong anisotropy of V O electronic structure Doping of TiO 2, ZnO WO 3 : photocatalytic water splitting Dopants induce formation of V O and shifts VB and CB edges or introduce localized levels in the gap (WO 3 ) 3 clusters on TiO 2 (110): heterogeneous catalysts TiO 2-x /WO 3 interaction (charge transfer) determines reaction mechanism

17 Models of real catalysts Concepts and interpretations

18 Au/TiO2 in CO oxidation at high temperature (FP7- CATSENSE project)

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20 Oxide catalysts in biomass conversion

21 Titania and zirconia in biomass conversion (FP7-CASCATBEL project) oxide surfaces Coke formation Ru, Ni particles r e a c t i o n Hydro-deoxygention ketonization H spillover nanoparticles O reverse spillover r e d u c t i o n H H 2 O desorption

22 EXTERNAL COLLABORATIONS AND PROJECTS HJ Freund N Nilius Fritz Haber MPI Berlin E Giamello Univ. Torino G Granozzi Univ. Padova J Behm Univ. Ulm Recent projects: P. Lievens Univ. Leuven MIUR PRIN New materials for CO2 and H2 cataysis MIUR FIRB Nanostructured oxides Cariplo Foundation Oxides photocatalysis DECORE FP Oxicarbides for fuel cells CASCATBEL FP Oxide catalysts for biofuels CATSENSE FP Metal clusters on oxides COST CM Reducible oxides

23 REVIEW ARTICLES

24 REVIEW ARTICLES

25 The University Milano Bicocca