Exploring the anomalous behavior of metal nanocatalysts with finite temperature AIMD and x-ray spectra

Transcription

1 Exploring the anomalous behavior of metal nanocatalysts with finite temperature AIMD and x-ray spectra F.D. Vila DOE grant DE-FG02-03ER15476 With computer support from DOE - NERSC.

2 Importance of Theoretical Simulations Why Theory? Access to structural and electronic properties Separation of local and global domains In particular, why Ab Initio Molecular Dynamics (AIMD)? Importance of non-equilibrium states Access to time-domain Outline: Importance of disorder in catalysis Dynamics and electronic structure in XANES Dynamics and structural disorder in EXAFS

3 Importance of Disorder in Catalysis

4 (R) XAFS: Access to Average Local Properties Cu K-edge FT of Ge EXAFS (k) XANES (1) R σ Leading cumulants σ 1 = r R 0 σ 2 T = r r 2 Multiple Scattering 2 σ R (Å) J. Kas et al. (2007) XANES: Access to average electronic structure EXAFS: Access to average bond distances and disorder

5 Early Success: EXAFS of Highly Dispersed Catalyst Experimental (XAS, STEM, TPR, and XPS) and Theoretical (DFT) Characterization of Supported Rhenium Catalysts S. Bare, S. Kelly, F. D. Vila, D. Boldingh, E. Karapetrova, J. Kas, G. Mickelson, F. Modica, N. Yang, J. J. Rehr J. Phys. Chem. C 115, 5740, 2011 DFT/EXAFS model with three species was used to identify the dominant Re adsorption site on the alumina surface. Re on g-al 2 O 3

6 Early Success: Explaining Anomalous NP Properties Dynamic structure in supported Pt nanoclusters: Real-time density functional theory and x-ray spectroscopy simulations F. D. Vila, J. J. Rehr, J. Kas, R. G. Nuzzo, A. I. Frenkel Physical Review B 78, (R), 2008 Librational Motion Complex dynamics: multiple-time scales, librational motion, fluctuating bonding 165 K Brownian-like Motion Simulations explain: large structural disorder, Negative Thermal Expansion (NTE). 573 K Pt 10 on g-al 2 O 3

7 New Concept: Dynamic Structural Disorder (DSD) DSD drives: Fluctuating bonding Cluster mobility Charge separation Layering and segregation Adsorbate dynamics (right) Adsorbate reactivity Inhomogeneity Rehr and Vila J. Chem. Phys. 140, (2014) CO dynamics on Pt 10 Sn 10

8 Disorder Affects Reactivity O 2 dissoc. on Pt 10 Sn 10 1 E act 2 3 Large differences in activation energy (E act ) Reaction path depends on DSD Initial St. Transition St. Final St.

9 Dynamics and Electronic Structure in XANES

10 Theory: Static Simulations are Inadequate +δ ( Oxidized ) -δ ( Metallic ) n Pt 10 on g-al 2 O 165 K MD simulations reproduce experiment Vila et al. Physical Review B 78, (R), 2008

11 Inhomogeneity in Well-defined(?) Nanoparticles On C On SiO 2 Bond contraction with heating/desorption White line: redshift, Emission line: blueshift EXAFS measurements: Predict truncated cuboctahedron Pt 37 Hypothesis: Both phenomena related to desorption Is inhomogeneity important to these phenomena too?

12 Charge Inhomogeneity CO-bound Pt atoms loose e each Layer charge alternation

13 Q Bond Length vs Charge Inhomogeneity R PtPt

14 Atomic Edge Absorption and Core Emission Shifts Opposite trends: Qualitatively reproduce experiment

15 Dynamics and Structural Disorder in EXAFS

16 Experiment: Anomalies in Supported Pt NPs σ 2 T k BT k s Negative Thermal Expansion (NTE) in smaller NPs Large 0K ( static ) disorder in smaller NPs Apparent bond strengthening with NP size decrease Sanchez et al. JACS 131, 7040, 2009

17 Anomalous Effective Grüneisen Parameter? γ = 1 3 d ln ν E d ln R PtPt Pt metal: Expt: g = 2.7 Theo: g = nm NPs: From Einstein Model Fit: Expt: g 5 2 γ 1 3 Δν E ΔR PtPt R PtPt ν E Einstein Model with Static Disorder σ 2 T = σ S 2 + h 8π 2 μ 1 ν e coth hν E 2k B T Effective Grüneisen parameter larger in NPs than bulk

18 ν E (THz) Anomalous Effective Grüneisen Parameter? γ = 1 d ln ν E γ 1 Δν E R PtPt 3 d ln R PtPt 3 ΔR PtPt ν E Pt metal: Expt: g = 2.7 Theo: g = nm NPs: From Einstein Model Fit: Expt: g ρ R ω N i=1 w i δ ν ν i ν E = ν = N i=1 wi ν i We can estimate from R-dependent PDOS

19 Anomalous Effective Grüneisen Parameter? Pt metal: Expt: g = 2.7 Theo: g = nm NPs: γ = 1 d ln ν E γ 1 Δν E R PtPt 3 d ln R PtPt 3 ΔR PtPt ν E From Einstein Model Fit: Expt: g 5 2 Pt 37 on C: From Vib. Component: g 1.7 What is the origin of this discrepancy?

20 Anomalous Bond Strength from Einstein Model Fits Einstein Model with Static Disorder σ 2 T = σ 2 S + h 1 8π 2 coth hν E μ ν e 2k B T 0.9 nm NPs: ν E 6.3 THz 1.1 nm NPs: ν E 4.6 THz ν E 6.4 THz ν E 6.4 THz ν E 4.5 THz Pt Foil: ν E 3.8 THz Sanchez et al. JACS 131, 7040, 2009

21 Computational Details Systems: Pt 10 and Pt 20 clusters Support: Cell: g-al 2 O 3 4 layers Dehydroxylated 19.4 Å 13.7 Å 16 Å vacuum MD Setup: 6 initial conditions 20 ps runs: 10 ps thermalization 10 ps analysis 3 fs time-step Nosé-Hoover thermostat Method: PBE XC functional US PPs, 297 ev cutoff VASP

22 Total Mean Square Relative Displacement (MSRD) σ 2 T = r r 2 = 1 τ 0 τ r t r 2 dt Reasonable agreement between theory and expt.

23 High (> 1THz) and Low (< 1 THz) Frequency Filtering r L t = + r τ F t τ dτ r H t = r t r L t Filter Function: F t = π 2 ν L cos πν L t, 0, t < 1 2ν L t 1 2ν L

24 Power Spectra of CM and Pt-Pt Dynamics dσ 2 dν = 2 π 0 r t r 0 cos 2πνt e εt2 dt r t r 0 = 1 τ 0 τr t + t r t dt Nice separation of slow and fast dynamic regimes

25 Vibrational MSRD ν E 4.1 THz 0.9 nm NPs: ν E 6.3 THz σ H 2 T = 1 τ 0 τ r H t r H 2 dt Pt Foil: ν E 3.8 THz Normal linear vibrational behavior

26 Dynamic Structural Disorder (DSD) MSRD σ L 2 T = 1 τ 0 τ r L t r L 2 dt Normal linear behavior: Low frequency quasi-harmonic modes

27 DSD: Correlation Between CM and Pt-Pt Dynamics Moderate/strong correlation between CM libration and Pt-Pt bonds

28 Quasi-static MSRD: Anomalous Structural Disorder σ 2 QS T = r L r L 2 Anomalous disorder (ASD): Causes apparent strengthening

29 Temp. Dep. Quasi-Static Bond Distributions (σ 2 QS T ) NTE Depleted Dynamic activation and depletion of long bonds

30 Effect of Support Support accounts for ~50% of disorder

31 Disorder in Pt 20 Support less important in Pt 20

32 Grüneisen Parameter: NPs vs Bulk γ = 1 3 d ln ν E d ln R PtPt Pt metal: Expt: g = 2.7 Theo: g = 2.8 Nanoparticle: From Einstein Model Fit: Expt: g 5 2 Theo: g 4 2 From Vib. Component: g γ 1 3 Δν E ΔR PtPt R PtPt ν E Grüneisen Parameter: Enhanced by anomalous disorder

33 Summary AIMD reveals: Importance of Structural Disorder: In catalysis In EXAFS and XANES analysis Single mechanism, dynamic activation, that explains: NTE Large disorder Bond strengthening Normal behavior of Pt-Pt vibrations, but slightly stronger bonds Coupling to CM motion Dynamic disorder Implications for interpretation of EXAFS: Analysis must account for both ASD and DSD Need new ASD modelling approach Anomaly signature: γ NP > γ Bulk

34 Exploring the anomalous behavior of metal nanocatalysts with finite temperature AIMD and x-ray spectra F.D. Vila Acknowledgements: J. J. Rehr J. J. Kas S. T. Hayashi S. Vilmolchalao A. F. Frenkel R. Nuzzo S. R. Bare DOE grant DE-FG02-03ER15476 With computer support from DOE - NERSC.