Catalytic Activity of IrO 2 (110) Surface: A DFT study

Transcription

1 Catalytic Activity of IrO 2 (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

2 Computational Chemistry in Surface Reactions in my lab Surface Characterization XRD, STM, IR, Raman, XPS, UPS Adsorption Character DOS, EDD Reaction Mechanism

3 Iridium dioxide is a metal oxide that exhibits metallic conductivity at room temperature. Its interesting electrochemical properties have led to IrO 2 being 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 IrO 2 ; this IrO 2 (110) surface has been characterized in recent studies, both experimentally and using density functional theory (DFT). * * 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,

4 The rutile structure- P4 2 /mnm(136) TiO 2,CrO 2, MnO 2, GeO 2, RuO 2, RhO 2, SnO 2, OsO 2, IrO 2, PbO XPS Study of IrO 2 (110) Surface 2. NH 3, N 2 and CH 4 Adsorption on IrO 2 Surface 3. NH 3 Oxidation on RuO 2 and IrO 2 Surfaces 4. CO 2 Capture on IrO 2 (110) Surface 5. C-H activation of CH 4 on IrO 2 (110) Surface

5 Both the RuO 2 and IrO 2 is crystallized in tetragonal rutile structure. On the stoichiometric (110) surface, it presents exposed rows of two-fold coordinated oxygen atoms (O br ), five-fold coordinated Ru atoms (M cus ; cus = coordinatively unsaturated site, M = Ru or Ir), and triply coordinated layer oxygen atoms (O 3f ), all along the [001] direction. Additional oxygen atoms may be adsorbed on top of the M cus atoms (so-called O cus ) through further exposure to O 2, then the surface is named the oxygen-rich surface. [ 110]( z) [ 001]( x) [ 1 10]( y) 5

6 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 (2 x 1)-MO 2 (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 RuO 2, and 14 Å for IrO 2. 6

7 NH x (x = 0 3 ), N 2 and CH 4 Adsorption [ 110]( z) [ 001]( x) [ 110]( y)

8 a b b E b = 2.14 ev d(ir N) = 2.09 Å d(n H a ) = 1.04 Å d(n H b ) = 1.03 Å d(o H a ) = 2.43 Å

9 [001] [1 10]

10 E b = 3.13 ev d(ir N) = 1.92 Å d(n H) = 1.02 Å

11 [001] [1 10]

12 E b = 3.29 ev d(ir N) = 1.84 Å d(n H) = 1.05 Å

13 [001] [1 10]

14 E b = 3.69 ev d(ir N) = 1.71 Å

15 [001] [1 10]

16 Binding Energy (ev), N N Stretching Frequency (cm -1 ) and Selected Geometry Parameters (Å) of N 2 Adsorption on s-ruo 2 (110) and s-iro 2 (110) Surfaces surface E b d(m N) a d(n N) b (N N) c s-ruo 2 (110) s-iro 2 (110) a M = Ru cus or Ir cus. b Calculated d(n N) in gas-phase is Å. c Calculated (N N) in gas-phase is 2398 cm -1. DOS of N 2 on IrO 2 (110) Surface

17 Analysis of Charge change ( q) Charge Change ( q) of N 2 Molecule and M cus (M = Ru, Ir) Atom in N 2 Adsorption Processes a RuO 2 IrO 2 M cus N * + 5 N u (sp) For N d (sp) the N 2 adsorption on the s-iro 2 (110) surface, more N u (p x +p y ) electrons are transferred to the surface The major contribution to the charge transfer comes N d (p x +p y ) N u from the bonding N d a The N u and N d is the upward and downward nitrogen atom in N 2 adsorption, respectively. The N d atom provides more electrons for the bond formation, and the N u atom contributes more electrons in bond formation.

18 d z The distribution of the vacant 2 state of the Ir cus 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 NH x species prior to adsorption. 18

19 b a E b = 0.48 ev d(ir C) = 2.56 Å d(ir H a ) = 1.86 Å d(c H a ) = 1.21 Å d(c H b ) = 1.15 Å d(o H b ) = 2.06 Å t a

20 b a [001] [1 10]

21 step E (ev) E (ev) IMF (cm -1 ) Dehydrogenation NH 3 -O cus i157 NH 3 -O br i218 NH 2 -O cus i897 NH 2 -OH cus i679 NH 2 -O br i255 NH 2 -O br i689 NH-O cus ~ NH-OH cus ~ NH-O br ~ NH-OH br i196 3.Ammonia Oxidation on RuO 2 (110) Surface H 2 O Formation OH cus -OH cus i232 OH cus -OH br i527 NO & N 2 Formation N cus -O cus i610 N cus -O br i568 N cus -N cus i579 H & N Diffusion H diff (cus-cus) i403 O br Row Ru cus Row H diff (cus-br) i872 H diff (br-br) i293 N diff (cus-cus) i Formation of N 2-cus and NO cus occurs on a single Ru cus row. 2. Increase the coverage of O cus atom will decrease the probability of N cus recombination and provide more O cus atoms for NO cus formation. Products Desorption H 2 O cus 1.22 H 2 O br 0.70 NO cus 2.09 * Wang et al., JPCB, 2005,109,7883

22 Ammonia Oxidation on IrO 2 (110) Surface Both the formation of NO 2-cus and N 2 O cus from NO cus has low reaction barrier. The production of NO from O br atom is not favored. Formation barrier of NO cus is lower than that of N 2-cus. 22

23 Ammonia Oxidation on IrO 2 (110) Surface 1. At low O cus coverage, there is no sufficient pathway O cus atoms for E NO E cus formation, IMF and N 2 NH x will Dehydeogenation be the major N-containing product. s-nh 3 -O br i As the increase of O cus coverage, the o-nh selectivity 3 -O br i686 to N 2-cus will rapidly decrease, o-nhand 3 -Oto cus N 2 O 0.44 cus will increase i229 s-nh 2 -O br i When the O cus coverage is high, the o-nh nitrogen 2 -O br i251 oxides (NO cus and NO 2-cus ) will o-nh become 2 -O cus i784 the main N-containing products. s-nh-o High br coverage 0.36 of O 0.08 cus will inhibit i166 the o-nh-o formation br of 0.08 N 2-cus and 0.46 N 2 O cus. i732 o-nh-o cus ~ i OH groups will inhibit the formation of N- containing product. Before the H 2 O desorbed from the surface, most of N 2-cus 1. and The Nbinding 2 O cus could energy not of be NH produced 3 on s-iro from 2 (110) the surface. is 2.14 ev. 2. Most of dehydrogenations are endothermic Due The barriers to the high of NH formation dehydrogenations energy of are H 2 O molecules, relatively lower the efficiency than others. of the ammonia oxidation on the IrO 2 (110) surface might be poor. step E (ev) E (ev) H 2 O Formation & H Diffusion OH cus -OH cus 0.42 OH cus -OH br 0.26 H diff (cus-cus) H diff (cus-br) H diff (br-br) N-Containing Products Formation N cus -N cus N cus -O cus N cus -O br NO cus -O cus N cus -NO cus Product Desorption H 2 O cus 1.57 N 2-cus 1.10 NO cus 2.16 NO 2-cus 2.00 N 2 O cus 0.83

24 Energy diagram of the adsorption and the first dehydrogenation of CH 4 on IrO 2 (110) surfaces. Energetics of the Adsorption and the First Dehydrogenation of CH 4 molecule on Different Catalytic Surfaces surface E ads (ev) E ( ev) E(eV) Rh(111) PdO(100) Li/MgO Ni(111) Pt(111) Ni(111) Ni(100) NiO(100) Ce (111) Sn/Ni(111) Ni(111) Ni(211) Bunnik, B. S.; Kramer, G. J. J. Catal. 2006, 242, Blanco-Reya, M.; Jenkins, S. J. J. Chem. Phys. 2009, 130, Zobela, N.; Behrendt, F. J. Chem. Phys. 2006, 125, Nave, S.; Jackson, B. J. Chem. Phys. 2009, 130, Haroun, M. F.; Moussounda, P. S.; Légaré, P. Catal. Today 2008, 138, Xinga, B.; Pang, X.-Y.; Wang, G.-C.; Shang, Z.-F. J. Mol. Catal. A. 2010, 315, Knapp, D.; Ziegler, T. J. Phys. Chem. C 2008, 112, Nikolla, E.; Schwank, J.; Linic, S. J. Catal. 2009, 263, 220.

25 Conclusions The DOS, EDD and q analyses provide detailed bond characters for the molecule adsorptions on IrO 2 (110) surface NH x dehydrogenation, H 2 O formation, and N-containing products formation are the three major processes in ammonia oxidation. Without the appearance of O cus 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 IrO 2 (110) surface. IrO 2 is good catalyst for C-H activation of methane, and might have potential for nitrogen fixation.

26 Acknowledgement NSC NCHC INER 林

27 Thanks for your attention!