Catalytic Activity of IrO2(110) Surface: A DFT study

• Jyh-Chiang Jiang• Department of Chemical Engineering, National

Taiwan University of Science and Technology (NTUST)

NCTS-NCKU 9/7, 2010

Computational Chemistry in Surface Reactions in my lab

• Surface Characterization XRD, STM, IR, Raman, XPS, UPS

• Adsorption CharacterDOS, EDD

• Reaction Mechanism

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• Iridium dioxide is a metal oxide that exhibits metallic conductivity at room temperature.

• Its interesting electrochemical properties have led to IrO2being used in several applications, such as electrode materials for chlorine or oxygen evolution, optical switching layers in electrochromic displays, pH-based electrodes and thin film electrode in ferroelectric capacitors.

• The (110)-oriented domain is one of the dominative surfaces of IrO2; this IrO2(110) surface has been characterized in recent studies, both experimentally and using density functional theory (DFT).*

About IrO2

* He, Y. B.; Stierle, A.; Li, W. X.; Farkas, A.; Kasper N.; Over, H. J. Phys. Chem. C 2008, 112, 11946; Chung, W.-H.; Wang, C.-C.; Tsai, D.-S.; Jiang, J.-C.; Cheng, Y.-C.; Fan, L.-J.; Yang Y.-W.; Huang, Y.-S. Surf. Sci. 2009, 604, 118.

The rutile structure- P42/mnm(136)

TiO2 , CrO2, MnO2, GeO2, RuO2, RhO2, SnO2, OsO2, IrO2, PbO2.

1. XPS Study of IrO2(110) Surface

2. NH3 , N2 and CH4 Adsorption on IrO2 Surface

3. NH3 Oxidation on RuO2 and IrO2 Surfaces

4. CO2 Capture on IrO2(110) Surface

5. C-H activation of CH4 on IrO2(110) Surface

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• Both the RuO2 and IrO2 is crystallized in tetragonal rutile structure. • On the stoichiometric (110) surface, it presents exposed rows of two-fold

coordinated oxygen atoms (Obr), five-fold coordinated Ru atoms (Mcus; cus = coordinatively unsaturated site, M = Ru or Ir), and triply coordinated layer oxygen atoms (O3f), all along the [001] direction.

• Additional oxygen atoms may be adsorbed on top of the Mcus atoms (so-called Ocus) through further exposure to O2, then the surface is named the oxygen-rich surface.

)](001[ x

)](101[ y

)](110[ z

[001] Row

OcusMcusObr

Surface Structure

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• Vienna Ab-initio Simulation Package (VASP)

• XC Functional: PW91, with GGA Approximation.

• Energy Cutoff: 300 eV for structural optimization, and 1000 eV for DOS calculations.

• K-points: 3x3x1, by Monkhost-Pack.• Including Spin Polarized Calculation.• All structures were checked by normal

mode frequency analysis.• TS determination: NEB

Calculation Parameters

• (2 x 1)-MO2(110) Surface.• 4 O－M－O Layers (12 Atomic Layers).• Fix Lower 5 Atomic Layers, and Relax

Upper 7 Atomic Layers. • 13 Å Vacuum Slab for RuO2, and 14 Å

for IrO2.

The Supercell

NHx (x = 0–3 ), N2 and CH4 Adsorption

)](001[ x

)](101[ y

)](110[ z

NH3 NH2

NH N

N2 CH4

NH3 Adsorption

Eb = –2.14 eVd(Ir–N) = 2.09 Åd(N–Ha) = 1.04 Åd(N–Hb) = 1.03 Åd(O···Ha) = 2.43 Å

a bb

]001[

]101[

NH3

EDD and PDOS Analyses

NH2 Adsorption

Eb = –3.13 eVd(Ir–N) = 1.92 Åd(N–H) = 1.02 Å

]001[

]101[

NH2

EDD and PDOS Analyses

NH Adsorption

Eb = –3.29 eVd(Ir–N) = 1.84 Åd(N–H) = 1.05 Å

EDD and PDOS Analyses

]001[

]101[

NH

N Adsorption

Eb = –3.69 eVd(Ir–N) = 1.71 Å

EDD and PDOS Analyses

]001[

]101[

N

N2 Adsorption

Binding Energy (eV), N–N Stretching Frequency (cm-1) and Selected Geometry Parameters (Å) of N2 Adsorption on s-RuO2(110) and s-IrO2(110) Surfaces

surface Eb d(M–N)a d(N–N)b (N–N)c

s-RuO2(110) –0.53 2.027 1.118 2280

s-IrO2(110) –1.10 1.934 1.124 2199a M = Rucus or Ircus.b Calculated d(N–N) in gas-phase is 1.111 Å.c Calculated (N–N) in gas-phase is 2398 cm-1.

DOS of N2 on IrO2(110) Surface

a The Nu and Nd is the upward and downward nitrogen atom in N2 adsorption, respectively.

0.5680.494Nd

0.6060.612Nu

0.0920.157Nd(px+py)

0.3090.321Nu(px+py)

0.4010.4781 + 2

0.4760.337Nd(sp)

0.2970.291Nu(sp)

0.7730.6283+ 4* + 5

1.1741.106N2

–0.378–0.007Mcus

IrO2RuO2

Charge Change (q) of N2 Molecule and Mcus (M = Ru, Ir) Atom in N2 Adsorption Processesa

Analysis of Charge change (q)

For the N2 adsorption on the s-IrO2(110) surface, more electrons are transferred to the surface.

The major contribution to the charge transfer comes from the bonding.

The Nd atom provides more electrons for the bond formation, and the Nu atom contributes more electrons in bond formation.

Nu

Nd‧‧

‧‧

Mcus

Nu

Nd

Mcus

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• The distribution of the vacant state of the Ircus atom is located at the lowest energy region, and is closer to the Fermi energy.

• This lower vacant state is closer to the highest occupied molecular orbitals (HOMOs) of the NHx species prior to adsorption .

2zd

CH4 Adsorption

Eb = –0.48 eVd(Ir–C) = 2.56 Åd(Ir–Ha) = 1.86 Åd(C–Ha) = 1.21 Åd(C–Hb) = 1.15 Åd(O···Hb) = 2.06 Å

ab

t

a

]001[

]101[

EDD Analysis

ab

step E‡ (eV) ΔE (eV) IMF (cm-1)DehydrogenationNH3-Ocus 0.71 0.62 i157NH3-Obr 0.44 0.43 i218NH2-Ocus 0.48 0.34 i897NH2-OHcus 0.31 0.31 i679NH2-Obr 0.86 0.11 i255NH2-Obr-2 0.59 0.43 i689NH-Ocus ~ 0 –0.82NH-OHcus ~ 0 –0.93NH-Obr ~ 0 –0.78NH-OHbr 0.48 0.47 i196

H2O FormationOHcus-OHcus 0.17 0.04 i232OHcus-OHbr 0.04 0.03 i527

NO & N2 FormationNcus-Ocus 0.47 –1.82 i610Ncus-Obr 0.89 0.03 i568Ncus-Ncus 0.20 –3.79 i579

H & N DiffusionHdiff(cus-cus) 0.09 0.00 i403Hdiff(cus-br) 0.17 0.03 i872Hdiff(br-br) 2.25 0.00 i293Ndiff(cus-cus) 1.44 0.00 i209

Products DesorptionH2Ocus 1.22

H2Obr×0.70

NOcus 2.09N2 0.53

* Wang et al., JPCB, 2005,109,7883

3.Ammonia Oxidation on RuO2(110) Surface

1. Formation of N2-cus and NOcus occurs on a single Rucus row.

2. Increase the coverage of Ocus atom will decrease the probability of Ncus recombination and provide more Ocus atoms for NOcus formation.

Obr Row Rucus Row

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N-Containing Products

Formation barrier of NOcus is lower than that of N2-cus.

The production of NO from Obr atom is not favored.

Both the formation of NO2-cus and N2Ocus from NOcus has low reaction barrier.

Ammonia Oxidation on IrO2(110) Surface

step E‡ (eV) ΔE (eV)

H2O Formation & H Diffusion

OHcus-OHcus 0.42

OHcus-OHbr 0.26

Hdiff(cus-cus) 0.12 0.00

Hdiff(cus-br) 0.37 0.32

Hdiff(br-br) 2.18 0.00

N-Containing Products Formation

Ncus-Ncus 0.63 –4.06

Ncus-Ocus 0.05 –1.56

Ncus-Obr 1.21 –0.02

NOcus-Ocus 0.67 0.24

Ncus-NOcus 0.51 –0.91

Product Desorption

H2Ocus 1.57

N2-cus 1.10

NOcus 2.16

NO2-cus 2.00

N2Ocus 0.83

1. At low Ocus coverage, there is no sufficient Ocus atoms for NOcus formation, and N2will be the major N-containing product.

2. As the increase of Ocus coverage, the selectivity to N2-cus will rapidly decrease, and to N2Ocus will increase.

3. When the Ocus coverage is high, the nitrogen oxides (NOcus and NO2-cus) will become the main N-containing products. High coverage of Ocus will inhibit the formation of N2-cus and N2Ocus.

4. OH groups will inhibit the formation of N-containing product. Before the H2O desorbed from the surface, most of N2-cusand N2Ocus could not be produced from the surface.

5. Due to the high formation energy of H2O molecules, the efficiency of the ammonia oxidation on the IrO2(110) surface might be poor.

Ammonia Oxidation on IrO2(110) Surface

pathway E‡ ΔE IMF

NHx Dehydeogenation

s-NH3-Obr 0.35 0.27 i257

o-NH3-Obr 0.57 0.53 i686

o-NH3-Ocus 0.44 0.32 i229

s-NH2-Obr 0.56 0.51 i340

o-NH2-Obr 0.68 0.58 i251

o-NH2-Ocus 0.41 0.16 i784

s-NH-Obr 0.36 0.08 i166

o-NH-Obr 0.08 –0.46 i732

o-NH-Ocus ~ 0 –0.66 i260

1. The binding energy of NH3 on s-IrO2(110) surface is –2.14 eV.

2. Most of dehydrogenations are endothermic.3. The barriers of NH dehydrogenations are

relatively lower than others.

5. C-H acivation of CH4

Energy diagram of the adsorption and the first dehydrogenation of CH4 on IrO2(110) surfaces.

Energetics of the Adsorption and the First Dehydrogenation of CH4 molecule on Different Catalytic Surfaces

surface Eads(eV) E‡(eV) E(eV)

Rh(111)1 0.72 0.09

PdO(100)2 1.58 –0.34

Li/MgO3 0.75

Ni(111)4 1.07

Pt(111)4 0.93

Ni(111)5 –0.37

Ni(100)6 –0.10 0.60 0.05

NiO(100)6 –0.06 1.64 1.18

Ce (111)7 1.44 –0.91

Sn/Ni(111)8 1.28

Ni(111)8 1.17

Ni(211)8 0.91

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Conclusions

• The DOS, EDD and q analyses provide detailed bond characters for the molecule adsorptions on IrO2(110) surface

• NHx dehydrogenation, H2O formation, and N-containing products formation are the three major processes in ammonia oxidation. Without the appearance of Ocus atoms, the ammonia oxidation could not occur.

• Although there is no available experimental data could be compared with, the predicted reaction mechanisms according to the DFT calculations provide an insight into possible results of the ammonia oxidation on the IrO2(110) surface.

• IrO2 is good catalyst for C-H activation of methane, and might have potential for nitrogen fixation.

Acknowledgement

• NSC • NCHC• INER• 林明璋院士

Thanks for your attention!