1 Institute of Isotopes, Budapest, Hungary; 2 Research Institute for Technical Physics and Materials Science, Budapest Hungary; 3 Chemical Physics of Materials, ULB, Brussels, Belgium Gold and silver catalysts for abatement of environmentally harmful materials: modelling the structure dependency Krisztina Frey 1, Gábor Pető 2, Viacheslav Iablokov 3, Norbert Kruse 3 and László Guczi 1

INTRODUCTIONINTRODUCTION Automotive exhaust contains high concentrations of NO x, CO and hydrocarbons, which are harmful to human health. All commercial solutions (Three Way Catalysis, Selective Catalytic Reduction, Storage, etc.) are based on platinum, palladium and rhodium metal. The present work is aimed at exploring the potential of a new generation of catalysts, based on gold and silver, to replace the more expensive platinum and rhodium metals.

GOAL of the present work Provide sound understanding of the intrinsic properties of silver and gold based model catalysts: 1.Compare activities CO oxidation between silver and gold supported catalysts. 2.Study gold particles and gold films either in presence or absence of metal-oxide supports (TiO 2 and FeO x ). What is the effect of metal-oxides? 3.Demonstrate the CO oxidation activity in sandwiched layers of gold/metal-oxide systems („inversed” system). 4.Use high resolution microscopy (AFM) and spectroscopy (SIMS, XPS) to characterize the morphology of the used layered catalysts.

Justification of the Approach  Unique electronic and morphological properties may develop when dimensions reach the nanoscale.  These changes may alter the chemical reactivity.  Develop an understanding of the key features of nanoscale materials is a first step towards the control and design of highly selective catalysts at otherwise optimum catalytic activity.

Ag/TiO 2 Sample preparation Ag(COO) 2 → Ag + 2 CO 2 oxalate co-precipitation Ag/TiO 2 catalysts have been prepared via oxalate co-precipitation of AgNO 3 and Ti(OCH(CH 3 ) 2 ) 4 as precursors than TPO (temperature- programmed oxidation). Ag loadings in the catalysts were 2, 4, 7 and 10% (w/w). TPO

Ag/TiO 2 Sample characterization (HR)TEM 4%Ag/TiO 2 10%Ag/TiO 2 Ag

In circulate system: 9 mbar CO and 9 mbar O 2 and 162 mbar He at 313 K Ag/TiO 2 CO oxidation in circulate system E a =13-25 kJ/mol

Ag/TiO 2 CO oxidation in flow system CO+O 2 mixture of 2% each, using Ar as diluent with a flow rate of 50 ml/min.

Au/SiO 2 /Si(100) catalysts

CharacterizationCharacterization TEM  TEM  particle size. AFM  AFM  particle size and surface roughness. SEM  SEM  scanning electron microscopy. SIMS, XPS  SIMS, XPS  surface composition. XPS, UPS  XPS, UPS  electron structure. CO oxidation  CO oxidation  catalytic activity. Effect examination: Characterization:

The samples were treated in 200 mbar H 2 at 573 K for 1 h, then at 803 K CO oxidation was performed in an all glass made circulation reactor connected to a QMS. A mixture of 9 mbar CO + 18 mbar O mbar He was used. The initial rate of CO 2 formation was related to the geometrical surface of the wafer (cc. 0.5 cm 2 ). The samples were treated in 200 mbar H 2 at 573 K for 1 h, then at 803 K CO oxidation was performed in an all glass made circulation reactor connected to a QMS. A mixture of 9 mbar CO + 18 mbar O mbar He was used. The initial rate of CO 2 formation was related to the geometrical surface of the wafer (cc. 0.5 cm 2 ). CO oxidation

Au/SiO 2 /Si(100) Sample preparation SiSiO 2 Au Implantation (40 keV, atom/cm 2 ) Evaporation h Au = 10 nm h Au = 80 nm

Modelling „inverse” FeO x /Au and TiO 2 /Au system 80 nm Au film/SiO 2 /Si(100)Au nanoparticles/SiO 2 /Si(100) Si Au FeO x, TiO 2 Laser ablation (PLD) SiO 2

FeO x /Au/SiO 2 /Si(100) CO oxidation In circulate system: 9 mbar CO, 18 mbar O 2 and 162 mbar He at 803 K

CO oxidation FeO x /Au/SiO 2 /Si(100) CO oxidation

CO oxidation TiO 2 /Au/SiO 2 /Si(100) CO oxidation

Au nanoparticles on SiO 2 /Si(100) AFM

80 nm Au film/SiO 2 /Si(100): AFM

Au characterization Bulk or nano? Au 4f XPS bulk nano

Si 2p XPS Au nanoparticles 80 nm Au film covered uncovered

FeO x /Au/SiO 2 /Si(100) 80 nm Au film/SiO 2 /Si(100) Au nanoparticles/SiO 2 /Si(100) SIMS SEM On the surface of Fe 2 O 3 covered Au/SiO 2 /Si(100) (Au nanoparticles & 80 nm thick Au film) no gold was detected.

XPS spectra of FeO x /Au layer/SiO 2 /Si(100) Fe 2 O 3 (Fe 3+ ) and FeO (Fe 2+ )

Effect of temperature used in CO oxidation on the surface: cracks ? nano Au FeO x /nano Au 80 nm Au FeO x /80 nm Au SEM after 5 min reaction 0.1  m

 Core level binding energy is shifted to higher value when gold film was transferred into gold nanoparticles.  When an “inverse” Au/FeO x is fabricated by FeO x deposition onto either Au nanoparticles/SiO 2 /Si(100) or 80 nm Au film/SiO 2 /Si(100) reference sample, the catalytic activity in the CO oxidation is enhanced compared to both Au/SiO 2 /Si(100) and FeO x /SiO 2 /Si(100).  The activity enhancement is larger for nano-type Au/FeO x than for bulk-type Au/FeO x co-operation. The gold effect is indirect, because Au is not exposed to the surface and it modifies the catalytically working FeO x.  The activity of the TiO 2 /SiO 2 /Si(100) was higher than that of TiO 2 /Au/SiO 2 /Si(100), which indicated an inhibiting effect of gold layer underneath the TiO 2 overlayer.  Ag/TiO 2 catalysts with different Ag loadings (2, 4, 7 and 10% (w/w)) the best conversion performance was obtained in a CO/O 2 =1:1 mixture over 10% Ag/TiO 2 for which the temperature of 50% CO conversion was T 50 =333 K We can conclude that

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Au nanoparticles on SiO 2 /Si(100): before and after CO oxidation before after J. Am. Chem. Soc., 125(14), (2003)

After CO oxidation BULK!!! 10 nm implanted Au/SiO 2 /Si(100) XPS spectrum 1h catalytic reaction! J. Am. Chem. Soc., 125(14), (2003) The aggregated Au particles shows „bulk type” electron structures.

FeO x /Au/SiO 2 /Si(100) Is FeO x porous? 10 nm FeO x /Au/SiO 2 /Si(100) 40 nm FeO x /Au/SiO 2 /Si(100) 40 nm FeO x /SiO 2 /Si(100) In case of thick FeO x layer no effect of underlying gold on the catalytic activity of FeO x.

AFM picture after 5 min. reaction 5 min. reaction AFM picture after 5 min. reaction 5 min. reaction