Dr. Ramesh Ch. Deka Department of Chemical Sciences, Tezpur University Assam Structural and electronic properties of Pd n and Au n clusters How do cluster properties scale with size? Relative energy, Bond length, binding energy, IP, EA, Chemical Hardness Cluster isomers Catalysis CO oxidation reverse hydrogen spillover The QM/MM approach for supported clusters Objective Molecules, Clusters, Solids Atoms, Molecules Elementary unit of a substance Discrete electronic spectrum Only "surface" atoms Solids Very large amount of elementary units Continous electronic spectrum Properties essentially independent of size Clusters Large number of elementary units Dense electronic spectrum Comparable amount of surface and bulk-like coordinated atoms (equal near 300 atoms) Properties depend on size Typical size range < molecule nm few atoms nm cluster < colloid < some 100 nm10 to some 10 6 atoms m microcrystallite = solid> 10 9 atoms Types of cluster Aggregate of n number of particles (atoms or molecules) with n = (or more) Homo-atomic or homo-molecular clusters : A n Examples : Pd n, Pt n clusters Hetero-atomic or hetero-molecular clusters : A n B n Example : Au n Pd n, Pt n Pd n clusters Different types of bonds result different clusters Cluster properties are significantly different from molecules and bulk material Cluster properties are size dependent and thus tunable Ionic nanoclusters (e.g. NaCl, ZnO) Strong ionic bonding, binding energy: 2-4 eV / atom Metal nanoclusters van der Waals nanoclusters (e.g. rare gas clusters) Weak bonding due to polarization, binding energy: 0.3 eV / atom Simple and noble metal nanoclusters (e.g. Na, Al, Ag) Transition metal nanoclusters (e.g. Fe, Pd) Semiconductor nanoclusters (e.g. Si, C, B) Strong covalent bonding, binding energy: 1-4 eV / atom Moderately strong metallic bonding, binding energy: eV / atom Moderate to strong metallo-covalent bonding, binding energy: eV / atom Examples of clusters Molecular nanoclusters (e.g. H 2 O clusters) Strong covalent bonding within the molecule + weak intermolecular forces Electronic structure of gas phase metal clusters The energy band structure in (a) bulk metal turns to discrete energy levels with opening of bandgap in (b) nanocluster of 100 atoms and (c) small metal cluster containing 3 atoms [ Introduction to Nanotechnology by Charles P. Poole Jr. and Frank J. Owens]. Reactivity Magnetism Melting temperature Stability Size dependent properties of gas phase metal nanoclusters Y. Chushak and L. S. Bartell, J. Phys. Chem. B 105, 11605, (2001) Gold clusters A.J. Cox et al. Phys. Rev. B 49, (1994) S.C. Richtsmeier et al. J. Chem. Phys. 82, 3659 (1985) W. D. Knight et al. Phys. Rev. Lett. 52, 2141 (1984) Literature review on oxide and zeolite supported palladium clusters CO Oxidation on a Single Pd Atom Supported on Magnesia, S. Abbet, U. Heiz, H. Hkkinen and U. Landman, Phys. Rev. Lett. 86, 5950 (2001) (Fig. 2) Global and non-local coupling in oscillating heterogeneous catalytic reactions : The oxidation of CO on zeolite supported palladium, M. M. Slinko, A. A. Ukharskii and N. I. Jaeger, Phys. Chem. Chem. Phys. 3, 1015 (2001) (Fig. 3) Stabilization of Metal Ensembles at Room Temperature: Palladium Clusters in Zeolites, J. Phys. Chem. 93, 6116, 1989 Fig. 1. Four Pd atoms located in a sodalite cage of faujasite zeolite Experimental work: Theoretical work: Reverse hydrogen spillover in supported subnanosize clusters of the metals of groups 8 to 11. A computational model study, Georgi N. Vayssilov and Notker R sch, Phys. Chem. Chem. Phys. 7, 4019 (2005) Zeolite-Supported Palladium Tetramer and its Reactivity towards H 2 molecules: Computational Studies, J. Moc, D. G. Musaev and K. Morokuma, J. Phys. Chem. A 112, 5973 (2008) The first question to answer in understanding cluster properties is what is the most stable cluster structure from an energetic point of view" small particle structures cannot be understood purely from experimental data and it is necessary to simultaneously use theoretical or other modeling [L. D. Marks, Rep. Prog. Phys. 57, 603, 1994] Motivation Gold clusters electronic devices - applications in materials science, medicine - catalysis Palladium clusters - catalysts in automotive exhaust systems Methodology All electron relativistic Density Functional calculations Functional used- BLYP, B3LYP Basis sets- DNP, 6-31G**, Program GAMESS DMol 3, GAUSSIAN Number of publications where the phrase density functional theory appears in the title or abstract (taken from ISI Web of Science) SystemPropertyExpt.RNRR-NRMethodRef. Au (atom)IP(eV) CCSD EA(eV) CCSD CCSD(T) Au + CCSD(T) AuHR e (pm) AE CCSD(T) PP CCSD(T) e (cm -1 ) AE CCSD(T) PP (CCSD(T) D e (eV) AE CCSD(T) PP CCSD(T) Au 2 R e (pm) AE cp-CCSD(T) e (cm -1 ) AE cp-CCSD(T) D e (eV) AE cp-CCSD(T) Latest relativistic benchmark results; AE= all- electron; PP=pseudopotential methods =polarizability in atomic unit, R e = bond lengths in pm, e vibrational frequencies in cm -1, De= dissociation energies in eV etc. MethodR e ( ) e (cm -1 )D e (eV) BLYP/DNP BOP/DNP BP/DNP HCTH/DNP PBE/DNP PW91/DNP rPBE/DNP VWN_BP VWN Experimental Bond length, vibrational frequency and dissociation energy of Au 2 at different levels The lowest-energy structures of gold cluster, Au n with n=2- 13 Planar to 3D transition: at n=7, Hakkinen & Landman, GGA, Phys. Rev. B, 62, R2287, 2000 at n=11, Walker, LDA, JCP, 122, 94310, 2005 at n =15, Wang et al, DFT (PW91), CPL, 392, 452, Deka & Deka Theochem, 870 (2008) 83-93 nShapeSymmetryTotal Energy (eV) Relative Energy (eV)E b (eV)D(eV) Linear D h Linear D h V C 2v Triangle D 3h Trapezoid C s Y(distorted) C s Zigzag C s Pyramidal T d W C 2v X D 2h Bipyramid D 3h Triangle C 2v Tricapped Triangle C s Pentagonal pyramid C 5v Rhombus C 2h Rhombus C Chair C 2h Boat C Capped Bipyramid C s Octahedron O h Triangle C 2h Zigzag C Stable clusters of Au n with n = 2 to 6 Deka & Deka Theochem, 870 (2008) 83-93 nShapeSymmetryTotal Energy (eV)Eb(eV)D(eV)IP(eV)EA(eV) Hardness (eV) Avg Bond Length() Linear D h Linear D h Trapezoid C s W C 2v Triangle C 2v Hexagon D 6h Capped Rhombus D 2h Capped Hexagon I C 2v Planar C 2v Planar C s Planar C 2v Planar C 2v Properties of most stable clusters of Au n with n = 2 to 13 Deka & Deka Theochem, 870 (2008) 83-93 Average bond length, binding energy per atom and fragmentation energy E b = nE(Au 1 )-E(Au n ) Binding energy per atom = E b /n The fragmentation energy: D(n,n-1) = E n-1 + E 1 E n nEb/n(eV)D(eV) Ave Coor. No Odd-even oscillation Even numbered clusters are more stable Variation of second energy differential of binding energy per atom and HOMO-LUMO gap with cluster size n 2 E(n) (eV) HOMO-LUMO Gap (eV) 2 E(n) = 2E(n) E(n+1) E(n-1) Even-numbered clusters have relatively higher values of 2 E(n) HOMO-LUMO gap important for electronic stability Large energy gap chemically more stable No correlations up to n = 5 Odd-even oscillation from n=6 to n=13 2 E b (n) = 2E b (n) E b (n+1) E b (n-1) Variation of IP, EA and chemical hardness with cluster size. n IP EA Even numbered clusters have higher VIP than the neighbouring odd numbered clusters VEA values also favour stability of even numbered clusters Even numbered clusters have higher hardness values Deka & Deka Theochem, 870 (2008) 83-93 Gas phase neutral, cationic and anionic palladium (Pd n, n=213) clusters Lowest energy structures of neutral, cationic and anionic Pd n (n=213) clusters Computational details for all gas phase calculations: DMol 3 program DNP basis set BLYP functional IP=E n + E n EA=E n E n - Kalita & Deka J. Chem. Phys. 127, 2007, Neutral clusters Point group Relative energy (eV) Multiplicity (M=1)Multiplicity (M=3)Multiplicity (M=5) Pd 1 Pd 2 D h Pd 3 D 3h Pd 4 CsCs TdTd D 4h Pd 5 D 3h C 2v Pd 6 C 2h OhOh D 3h Pd 7 D 5h CsCs C2C Pd 8 C 2v Cs C2C Pd 9 D 3h C 4v CsCs Pd 10 D 2h C 2v D 4d C 3v Pd 11 D 3h CsCs C 2v Pd 12 CsCs C 2v C1C Pd 13 C1C C 2v CsCs Relative stability of different structural and spin isomers of neutral Pd 1 Pd 13 clusters Cationic clusters Relative energy (eV) Anionic clusters Relative energy (eV) Multiplicity (M=2) Multiplicity (M=4) Multiplicity (M=6) Multiplicity (M=2) Multiplicity (M=4) Multiplicity (M=6) Pd Pd Pd 2 (D h ) Pd 2 (D h ) Pd 3 (D 3h ) Pd 3 (D 3h ) Pd 4 (T d ) Pd 4 (T d ) Pd 5 (C 4v ) Pd 5 (C 4v ) Pd 6 (O h ) Pd 6 (O h ) Pd 7 (D 5h ) Pd 7 (D 5h ) Pd 8 (D 2d ) Pd 8 (D 2d ) Pd 9 (D 3h ) Pd 9 (D 3h ) Pd 10 (D 2h ) Pd 10 (D 2h ) Pd 11 (D 3h ) Pd 11 (D 3h ) Pd 12 (C s ) Pd 12 (C s ) Pd 13 (C 1 ) Pd 13 (C 1 ) Relative stability of different structural and spin isomers of cationic and anionic Pd 1 Pd 13 clusters Kalita & Deka J. Chem. Phys. 127, 2007, Increment of average bond lengths from n=4 onwards BE of the clusters increases with cluster size Neutral Pd 4, Pd 6 and Pd 8 clusters are relatively stable There is no strong even-odd oscillation neutral Pd 4 is found to be the most stable cluster Kalita & Deka J. Chem. Phys. 127, 2007, Neutral Cationic Lowest energy structures of neutral Pd n CO (n=17) complexes Anionic CO adsorption on gas phase Pd n (n=17) clusters Lowest energy structures of cationic Pd n CO (n=17) complexes Lowest energy structures of anionic Pd n CO (n=17) complexes Red-oxygen, gray-carbon, blue-palladium Kalita & Deka Eur. Phys. J. D. 53, 2009, 51-58 Calculated parameters of neutral Pd n CO (n=17) complexes ComplexGeometrySpin state BE of CO (eV) Pd-Pd bond length () Pd-C distance () C-O bond length () C-O freq (cm -1 ) Pd 1 COOn-topSinglet1.859 Pd 2 COBridgeSinglet Pd 3 COThree-foldSinglet Pd 4 COThree-foldSinglet Pd 5 COOn-topSinglet Pd 6 COA-topSinglet Pd 7 COBridgeSinglet Kalita & Deka Eur. Phys. J. D. 53, 2009, 51-58 ComplexGeometrySpin state BE of CO (eV) Pd-Pd bond length () Pd-C distance () C-O bond length () C-O freq (cm -1 ) Pd 1 CO + On-topDoublet1.724 Pd 2 CO + BridgeDoublet Pd 3 CO + Three-foldDoublet Pd 4 CO + Three-foldDoublet Pd 5 CO + Three-foldDoublet Pd 6 CO + A-topDoublet Pd 7 CO + BridgeDoublet Calculated parameters of cationic Pd n CO (n=17) complexes Kalita & Deka Eur. Phys. J. D. 53, 2009, 51-58 ComplexGeometrySpin state BE of CO (eV) Pd-Pd bond length () Pd-C distance () C-O bond length () C-O freq (cm -1 ) Pd 1 CO - A-topDoublet1.538 Pd 2 CO - On-topDoublet Pd 3 CO - A-topDoublet Pd 4 CO - A-topDoublet Pd 5 CO - A-topDoublet Pd 6 CO - A-topDoublet Pd 7 CO - BridgeDoublet Calculated parameters of anionic Pd n CO (n=17) complexes Kalita & Deka Eur. Phys. J. D. 53, 2009, 51-58 Molecular and dissociative adsorption of oxygen on Pd 4 clusters Neutral complexes Cationic complexes Anionic complexes Geometry Multiplicity (M) O-O bond length () BE of O 2 (eV)O-O freq (cm -1 ) Mulliken charge on O 2 Pd 4 O TS32.06 Pd 4 2O Pd 4 O TS21.60 Pd 4 2O Pd 4 O TS42.03 Pd 4 2O Calculated parameters of Pd 4 O 2 complexes Kalita & Deka, J. Comput. Chem. (minor revision) ComplexMultiplicity (M) BE of O (eV) Pd-Pd bond length () Pd-O bond length () Mulliken charge on O Neutral Cationic Anionic Atomic oxygen adsorbed Pd 4 complexes Lowest energy structures of co-adsorption of CO and oxygen on Pd 4 clusters Neutral Cationic Anionic Kalita & Deka, J. Am. Chem. Soc. 131 (2009) Calculated parameters of oxygen and CO co-adsorbed Pd 4 complexes ComplexGeometry O-O bond length () C-O bond length () BE of CO (eV) O-O freq (cm -1 ) C-O freq (cm -1 ) Mulliken chargen on O 2 Mulliken charge on CO Neutral Pd 4 O 2 CO Pd 4 2OCO Pd 4 OCO 1996.60.02 Cationic Pd 4 O 2 CO Pd 4 2OCO Pd 4 OCO + 2080.00.16 Anionic Pd 4 O 2 CO Pd 4 2OCO Pd 4 OCO - 1884.6-0.13 Kalita & Deka, J. Am. Chem. Soc. 131 (2009) Complex BE of CO 2 (eV) Pd-Pd bond length () Pd-C bond length () C-O bond length () O-C-O angle (degree) Mulliken charge on CO 2 Neutral Pd 4 CO Cationic Pd 4 CO Anionic Pd 4 CO Neutral Pd 4 OCO Cationic Pd 4 OCO Anionic Pd 4 OCO CO 2 adsorbed and O, CO 2 co-adsorbed Pd 4 complexes Kalita & Deka, J. Am. Chem. Soc. 131 (2009) The reaction proceeds via following steps: Pd 4 +O 2 Pd 4 O2 (-0.82, -0.38, eV) Pd 4 +O 2 +CO Pd 4 O 2 CO (-2.27, -1.72, eV) Pd 4 +O 2 +CO Pd 4 OCO 2 (-3.96, -4.03, eV) Pd 4 +O 2 +CO Pd 4 O+CO 2 (-3.56, -3.57, eV) The energetic profile of complete reaction cycle for CO oxidation on neutral, cationic and anionic Pd 4 clusters: molecular oxygen reacting with CO Catalytic cycle of CO oxidation on neutral, cationic and anionic Pd 4 clusters: atomic oxygen reacting with CO Pd 4 +O 2 +COPd 4 O+CO 2 Kalita & Deka, J. Am. Chem. Soc. 131 (2009) The reaction proceeds via following steps: Pd 4 +O 2 Pd 4 O 2 (-0.82, -0.38, eV) Pd 4 +O 2 Pd 4 2O (-1.63, -1.21, eV) Pd 4 +O 2 +CO Pd 4 2OCO (-3.28, -2.68, eV) Pd 4 +O 2 +CO Pd 4 OCO 2 (-3.96, -4.03, eV) Pd 4 +O 2 +CO Pd 4 O+CO 2 (-3.56, -3.57, eV) The energetic profile of complete reaction cycle for CO oxidation on neutral, cationic and anionic Pd 4 clusters: dissociated oxygen reacting with CO Catalytic cycle of CO oxidation on neutral, cationic and anionic Pd 4 clusters: atomic oxygen reacting with CO Pd 4 +2O+COPd 4 O+CO 2 Kalita & Deka, J. Am. Chem. Soc. 131 (2009) The energetic profile of complete reaction cycle for CO oxidation on neutral, cationic and anionic Pd 4 clusters: atomic oxygen reacting with CO Catalytic cycle of CO oxidation on neutral, cationic and anionic Pd 4 clusters: atomic oxygen reacting with CO Pd 4 +O+COPd 4 +CO 2 The reaction proceeds via following steps: Pd 4 + O 2 +CO Pd 4 OCO Pd 4 + O 2 +CO Pd 4 CO 2 Pd 4 + O 2 +CO Pd 4 +CO 2 Pd 4 + O 2 Pd 4 O (-0.66, -0.67, eV) (-2.25, -2.25, eV) (-2.81, -3.07, eV) (-2.89, -2.89, eV) (1)Dissociative adsorption of oxygen is preferable on all the clusters. (2)Pre-adsorption of oxygen affect the adsorption geometry as well as the binding energy of CO in the co-adsorption complexes. (3)Oxygen adsorbed anionic Pd 4 cluster has stronger binding with CO followed by neutral and cationic clusters. (4)The reactants and intermediates of CO oxidation reaction have stronger binding with anionic Pd 4 than neutral and cationic clusters. (5)Neutral and cationic Pd 4 clusters are more effective in catalyzing CO oxidation than anionic cluster. (6)Dissociated oxygen is a superior oxidant than molecular and atomic forms. Kalita & Deka, J. Am. Chem. Soc. 131 (2009) Very recently experimental study of the reaction of CO with O 2 catalyzed by Pd n, n=1, 2, 4, 7, 10, 16, 20 and 25) clusters in gas phase and deposited on TiO 2 (110) has been performed. Evolution of Metal-Zeolite Interaction Modeling Bare cations Alumina tetrahedron Zeolite rings and chains variation of Al content variation of the ring size Embedded clusters (QM/MM) active site at QM level environment at MM level EXAFS, Gates et al. J. Phys. Chem. B, 108 (2004) XANES oxidation states of Au Mononuclear Au in HY zeolite Experimenta l results DFT Au (III)Au (I)Au (0) Au - O 2.21 2.25 2.06 2.44 Au - Al 3.20 3.08 3.06 3.35 q(Au) Hirshfeld q(Au) Mulliken 43 Oxidation of the Cluster by Surface OH Groups Rh 6 cluster supported on zeolite fragment Non-reactive adsorption Protons in OH groups of zeolite Reactive adsorption Protons move to Rh 6 cluster Reactive adsorption preferred Structure more stable by ~120 kJ/mol per transferred proton Chemical features of supported clusters Non-reactive adsorption of cluster: Polarization of electron density weakly cationic and anionic Rh centers Reactive adsorption of cluster: Oxidation of Rh centers interacting with support to Rh z, q(Rh z ) = 0.74 e Vayssilov, Gates, Rsch Angew. Chem. Int. Ed. Engl. 42 (2003) 1391 CovEPE Adsorption of Rh 6 in Faujasite Reactive adsorption, Rh 6 (3H)/Zeo Embedded structures are slightly closer to experimental data Embedding reduces proton transfer energy from 120 to 85 kJ/mol Non-reactive adsorption, Rh 6 /Zeo(3H) Gas-phase Pd 4 cluster Singlet energy Triplet energy= kJ/mol 2.58 Electronic state 1 A Point group C 1 Electronic state 3 A Point group C 2.58 2.83 2.61 2.71 2.72 Full optimization at B3LYP/LANL2DZ level in Gaussian 03 Top view of Zeo1(1H) Side view of Zeo1(1H) Pd 4 /Zeo1(1H) Pd 4 H/Zeo1 ONIOM2 (B3LYP/6-31G(d,p), LANL2DZ: UFF) optimized structures Kalita & Deka, J. Phys. Chem. C 113 (2009) Calculated parametersComplexes Zeo1(1H)Pd 4 /Zeo1(1H)Pd 4 H/Zeo1 Average Distances () AlO AlO SiO SiO Al(O1)Si Al(O2)Si PdzO1 PdzO2 PdzSi PdzAl PdzH/PdtH1.87/1.69 O1H Charges Pd t H Energies (kJ/mol) Eads-48.75 E RS Etrans Important structural parameters, Mulliken charges and enegetics of Zeo1(1H) and its complexes with Pd 4 Top view of Zeo2 (2H)Side view of Zeo2 (2H) Pd 4 /Zeo2 (2H)Pd 4 H/Zeo2 (1H) Pd 4 H 2 /Zeo2 (B3LYP/6-31G(d,p), LANL2DZ: UFF) optimized structures Kalita & Deka, J. Phys. Chem. C 113 (2009) Calculated parametersComplexes Zeo2(2H)Pd 4 /Zeo1(2H)Pd 4 H/Zeo1(1H)Pd 4 H 2 /Zeo1 Average Distances () AlO AlO SiO SiO Al(O1)Si Al(O2)Si PdzO1 PdzO2 PdzSi PdzAl PdzH/PdtH 1.86/ /1.72, 1.66/1.66 O1H Charges Pd t H / / /0.07 Energies (kJ/mol) Eads -74.09 E RS E trans Important structural parameters, Mulliken charges and enegetics of Zeo2(2H) and its complexes with Pd 4 Top view of Zeo3 (3H) Side view of Zeo3 (3H) Pd 4 /Zeo3 (3H) Pd 4 H/Zeo3 (2H)Pd 4 H 2 /Zeo3 (1H)Pd 4 H 3 /Zeo3 ONIOM2 (B3LYP/6-31G(d,p), LANL2DZ: UFF) optimized structures Kalita & Deka, J. Phys. Chem. C 113 (2009) Calculated parametersComplexes Zeo3(3H)Pd 4 /Zeo3(3H)Pd 4 H/Zeo3(2H)Pd 4 H 2 /Zeo3(1H)Pd 4 H 3 /Zeo3 Average Distances () AlO AlO SiO SiO Al(O1)Si Al(O2)Si PdzO1 PdzO2 PdzSi PdzAl PdzH/PdtH 1.80/ /1.71, 1.62/ /1.68, 1.65/1.70, 1.52/2.64 O1H Charges Pd t H 0.38/0.38/ /0.39/ /0.41/ /0.03/ /0.05/0.07 Energies (kJ/mol) Eads -72.65 E RS E trans Important structural parameters, Mulliken charges and enegetics of Zeo3(3H) and its complexes with Pd 4 Oxidation of the Au 6 Cluster by Surface OH Groups Au 6 cluster supported on zeolite fragment Non-reactive adsorption Protons in OH groups of zeolite Reactive adsorption Protons move to Au 6 cluster Reactive adsorption preferred Structure more stable by ~60 kJ/mol per transferred proton Chemical features of supported clusters Non-reactive adsorption of cluster: Polarization of electron density weakly cationic and anionic Au centers Reactive adsorption of cluster: Oxidation of Au centers interacting with support to Au z, q M (Au z ) = 0.16 e Deka, Deka & Miyamoto Catal. Lett. 131 (2009) Expt. Au-O ~ 2.08 2.14 , Langmuir 22 (2006) 4311 Deka, Deka & Miyamoto Catal. Lett. 131 (2009) Summary Different initial guess geometries lead to a number of structural isomers for each cluster size Most stable isomer of each of gold cluster with n 13 has planar geometry Odd-even oscillations are evident in HOMO- LUMO gap, second energy differences, binding energy per atom, IP, EA and hardness values Calculated results are comparable with the available experimental data Au 6 is most stable cluster - lowest energy structure of Pd 4 cluster is triplet Adsorption of Pd 4 cluster in zeolite fragment does not change the structure significantly Presence of support is affective in producing internal charge polarization of the cluster Pd 4 cluster undergoes partial oxidation due to proton transfer processes, the metal atoms closer to the zeolite oxygens are more oxidized than those located further away Pd 4 H/Zeo(mH),m=13 complexes with only one proton transfer from Zeolite support to Pd 4 cluster is the most stable species in triplet states Dissociated oxygen is a superior oxidant for CO oxidation than molecular and atomic oxygen Pd 4 + and Pd 4 are found to be more effective for catalyzing CO oxidation in comparison with Pd 4 - Acknowledgment Dr. Paritosh Mondal (AU) Kalyan K. Hazarika Ajanta Deka Bulumoni Kalita Pubalee Sarmah Rasna Devi Subhi Baishya Kusum Bania CSIR, DST and Tezpur University