Ab initio study of methane adsorption on PdPt oxide structures

Ab initio study of methane adsorption on PdPt oxide structures Arezoo Dianat Methane_Fuelcell.pdf

Institute for material science

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Motivation Interest in using methane (CH4) as alternative to coal and oil High H/C ratio

Heat generation with less CO2 emission

Problem: Methane combustion requires high temperature ( >1300 °C) to maintain stable flame undesired NOX production Solution: Catalytic combustion of methane at lower temperatures Methane_Fuelcell.pdf Minimization of NOX emissions Highly active catalysts for CH4 combustion: Metallic palladium and platinum Palladium oxide Palladium oxide supported by Pt

Palladium (Pd) and Platinum (Pt)

Increasing catalytic activity

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Experimental findings on Pt supported PdO catalyst in CH4 combustion Time-course of catalytic activities of Pt supported PdO catalysts

PdO + Pt

Increase of catalytic activity Preventing catalyst deactivation Why PdO + Pt better?

PdO

Methane_Fuelcell.pdf

623 K for 6 h

K. Narui, et al., Applied catalysis A: General 179 (1999) 165

Our aim: Understanding of catalytic activity of bimetal PdPt and its oxide phases for CH4 combustion 3

Exploration of stable phases of metal + adsorbate Example of monometallic Phase diagram of Pd-O-CO


J. Rogal, et al., Phys. Rev. Lett. 75 (2007) 2054331

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Exploring adsorption on Pd-Pt alloy system Oxygen

?

Oxygen adsorption energy on different Pd-Pt compositions and configurations

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Determination of stable oxide structures

Methane

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Dissociative adsorption energy of methane on Pd-Pt oxide structures CH4 (gas)

CH3 + H (solid surface)

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Exploring adsorption on Pd-Pt alloy system Oxygen

?

Oxygen adsorption energy on different Pd-Pt compositions and configurations

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Determination of stable oxide structures

Methane

?


Dissociative adsorption energy of methane on Pd-Pt oxide structures CH4 (gas)

CH3 + H (solid surface)

Pd-Pt bimetal better than pure Pd or Pt ?

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Outline Investigation of oxide structures

Methane adsorption

Oxygen adsorption

CH4

CH3 + H

O atoms Pd or Pt atoms

Thin oxide film


Substrate

Bulk oxide

PdO

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Computational methods Ab initio calculation based on density-functional theory - quantum mechanical theory to determine the electronic structure (ground state) of many body systems Motion of electrons described by Schrödinger equation and nuclei motion by classical equation of motion Forces acting on atoms evaluated using Hellman-Feynman theorem: Methane_Fuelcell.pdf

Coupling of electronic variables and nuclei motion

high computational cost

We use parallel version of widely used DFT code Vienna ab-initio simulation package (VASP)

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Computational cost High number of plane waves in wave function expansion ( > 10,000) Slab

Number of atoms: 24-60 Methane_Fuelcell.pdf

Number of electrons per atom: 10 About 550 - 1000 h CPU time for each calculation

Simulation cell with periodic boundary conditions

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Flow chart of a self-consistent calculation Chose elements and positions of ions

Cutoff of plane-wave basis set {e

i ( k +G ) r

}

Pick a trial density n(r )

Calculate VH (n) and VXC ( n)


 h2 2  ∇ + Vion + VXC + VH  ψ = Eψ Solve Hψ =   2m  by diagonalization of H K + G, K + G' Calculate new n(r ) Compute total energy

Yes

Is the solution self-consistent?

No

Generate new

n(r ) 10

Oxygen adsorption on Pd, Pt and PdPt (111) surfaces High symmetry on-surface sites

Most favorable on-surface site oxygen metal


High symmetry sub-surface sites

Most favorable sub-surface site 11

Chemisorbed oxygen on PdPt(111) Influence of PdPt configuration on oxygen adsorption energy

Methane_Fuelcell.pdf Eads= - [EO@S - ES - 1/2NOEO2] / NO

oxygen Pd Pt

Layer configuration leading to high adsorption energy

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Oxygen adsorption in sub-surface Influence of PdPt configuration on oxygen adsorption with sub-surface incorporation Mean adsorption energy for 1 ML oxygen coverage

Methane_Fuelcell.pdf Higher oxygen binding on mixed PdPt

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Thin mixed oxide film of Pt3O4 structure on PdPt(100) Oxygen binding energy

Lattice constant of Pt3O4 (5.63 Å) is almost exactly twice the Pt-Pt distance (2.82 Å)

PdO4/3 film Pd0.67Pt0.33O4/3 film


Pd0.33Pt0.67O4/3 film mixed PdPt

PtO4/3 film

PtO4/3 on Pt(100)

Pd-doping to Pt3O4-thin film on Pt(100) destabilizes the structure 14

Bulk Oxides Pd-Pt-O No experiment data on mixed PdPt oxides

Use known Pd and Pt oxide structures as base for mixed oxides

Mixed bulk Pd-Pt-O: Consider Pd-Pt compositions ¼ , ½ , ¾ Find favorable atomic configuration for each Pd/Pt ratio Favoured configurations for Pd:Pt ratio 3:4


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Stability range of PdPt bulk oxides at 1 atm pressure


PdxPt1-xO2 is the most stable phase at low temperature

At higher temperature, mixed phase decomposes to monometallic oxides PdO and PtO2

Increasing temperature, monometallic oxide of PdO and Pt3O4 becomes stable

For T>950 K, PdO-Pt3O4 oxide mixture decomposes into metallic PdPt

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Dissociative adsorption of methane CH4 (gas) CH3 + H (on surface) Pt(111)

Pd(111)

Eads = 0.19 eV

Methane_Fuelcell.pdf Eads = - 0.08 eV

endotherm

PdPt(111)

Eads = 0.20 eV endotherm

Eads = ECH3+H@S - ES - ECH4 Most stable adsorption sites:

CH3 on top-site H on fcc-site

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Methane adsorption on oxygen ad-layer

exotherm Methane_Fuelcell.pdf

CH3 H

Much higher methane adsorption energy on oxide structure compared to metal

Interaction of CH3 + H with substrate results in strong reconstruction of oxide surface 18

Methane adsorption on mixed PdPt thin oxide films on PdPt(100)

PtO4/3 on Pt(100)


Pd-doping of Pt3O4-thin oxide film

higher CH4 adsorption energy

Methane adsorption on thin oxide film is stronger than on oxygen ad-layer 19

Summary Study of methane adsorption on PdPt and Pd-Pt-O structures as base for understanding their catalytic activity for methane combustion Exploration of Pd-Pt oxide structures Methane adsorption energy Metallic Pd and Pt Oxygen ad-layer on pure Pd and Pt Thin Pd-oxide and Pt-oxide films Methane_Fuelcell.pdf

Increase

Oxygen ad-layer on mixed PdPt Thin mixed oxide film on PdPt

Ab-initio study suggests: Mixed PdPt-oxide structures are favoured for catalytic methane combustion

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Acknowledgments Manfred Bobeth (TU Dresden) Lucio Colombi Ciacchi (Fraunhofer Institut Freiburg) Gianaurelio Cuniberti (TU Dresden) Wolfgang Pompe (TU Dresden) Nicola Seriani (Universität Wien) Methane_Fuelcell.pdf Center for information services and high performance computing

DFG joint project with L. C. Ciacchi Ab-inito Modellierung der Oxidation bimetallischer Legierungen

Thank you for your attention 21

Publications 1. A. Dianat, J. Zimmermann, N. Seriani, M. Bobeth, W. Pompe, L. C. Ciacchi, “Ab inito study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces”, Surf. Sci. 602 (2008) 876.

2. A. Dianat, N. Seriani, M. Bobeth, W. Pompe, L.C. Ciacchi, “DFT study of the thermodynamic stability of Pd-Pt bulk oxide phases”, J. Phys. Chem. C 112 (2008) 13623. Methane_Fuelcell.pdf

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