Environmental and Energy Catalysis by Nanostructures

Transcription

1 Environmental and Energy Catalysis by Nanostructures Nanostructured catalysts already widely used for: Pollutant clean-up: e.g., catalytic converter in autos - probably prevented more cancer death that all medical treatment combined Chemicals production with less energy consumption, less polluting side products. Fuel conversions to more efficient, cleanly-burning forms. Promising future: Nanostructures now can be made with tuned reactivity, and far more homogenous in structure- key to high selectivity! So soon we could have (with proper research) lots better catalysts, thus: cleaner cars, chemical industry, power plants, heating. But nanostructures are inherently unstable wrt larger structures, leads to catalyst deactivation, slower catalyst development.

2 Electronic character of nanoparticles are tunable, with size as knob. Different colors for different sizes of nanoparticles of same semiconductor (Bawendi, Brus, Alivisatos, Nie, others). Bright idea: catalytic properties also ought to be tunable for metal nanoparticles.

3 Implications of Particle Size wrt: Chemisorption and Catalytic Reactivity Atoms of same element which are more coordinatively unsaturated (i.e., have fewer neighbors) tend to bind next species more strongly. Example: Bond energy between 2 C atoms increase as the number of H or R neighbors decreases: H 3 C CH 3 90 kcal/mol H 2 C CH kcal/mol HC CH 200 kcal/mol. Metal surface atoms in particles <4 nm in diameter have fewer neighbors and should be much less noble, and behave more like elements up and to left in periodic table. Should be able to tune catalytic properties with particle size rather strongly below 6 nm. Indeed this has been seen

4 Gold Nanoparticles on TiO 2 (110) Model of Au / TiO 2 catalysts for: Low-temperature CO oxidation (exhaust cleanup). Selective oxidations (e.g., of propene). from: M. Valden, X. Lai and D.W. Goodman, Science 281, (See also our related work of Murata group referenced there.) 2 nm Au particles = very active!!! 10 nm Au particles = completely inactive

5 We Study Model Oxide-Supported Metal Catalysts Vapor-deposited metals on single-crystal oxides: Simpler, structurally well-defined samples: clean surfaces, controlled particle sizes. Issues: Effect of metal particle dimensions on: turnover frequency, selectivity, chemisorption of intermediates. Electronic effects due to interaction with underlying oxide. Effect of oxide or crystal face on activity, resistance to sintering. Sintering mechanisms, kinetics. Strength of metal - oxide bonding. Reviews of approach: H J Freund, Faraday Disc. 114 (1999) 1. C R Henry, Surface Sci. Rept. 31 (1998) 231. D W Goodman, D Ranier, J. Mol. Catal. 131 (1998) 259. C T Campbell, Surface Sci. Rept. 27 (1997) 1.

6 STM Images from Bäumer and Freund group:

7 Gold Nanoparticles on TiO 2 (110) Model of Au / TiO 2 catalysts for: Low-temperature CO oxidation (exhaust cleanup). Selective oxidations (e.g., of propene). from: M. Valden, X. Lai and D.W. Goodman, Science 281, (See also our related work of Murata group referenced there.) 2 nm Au = very active!!! 10 nm Au = completely inactive

8 2 O gas 2 O ad O 2,gas Possible explanation for why Au nanoparticles more active in oxidation catalysis: E a,des and H ad for O ad : ~40% larger for smallest Au particles

9 O 2 / Au / TiO 2 (110) Difference in E des = 34 kj/mol Difference in E a = 34 kj/mol if β = 0.5 in Bronsted relation (large) Au islands = 68 kj/mol if β = 0.5 in Bronsted relation (tiny) Au islands Au particle size effect O 2 -Au energy diagram V. Bondzie, S. C. Parker and C. T. Campbell, Catalysis Letters 63 (1999) 143.

10 Studies related to selective propene epoxidation on nano-au / TiO 2 H. Ajo et al. Catalysis Letters 78 (2002) 359

11 MgO(100) thin film (~4.0 nm thick) grown on 1 µm-thick Mo(100) Following recipe from: D. W. Goodman, Chem. Phys. Lett. 182 (1992) 472.

12 Metal adsorption on MgO(100)/Mo(100) 350 Heat of adsorption [kj/mol] Cu on MgO(100) Ag on MgO(100) Pb on MgO(100) Cu H Sublimation 337 kj/mol Ag H Sublimation 285 kj/mol Pb H Sublimation 195 kj/mol Metal coverage [ML] Cu: J. T. Ranney et al., Faraday Discussions, 114, 195, Ag: J. H. Larsen et al., Phys. Rev. B 63, , Pb: D. E. Starr et al., J. Chem. Phys. 114, , 2001.

13 Cu BE(Cu-MgO) = 198 kj/mol MgO(100) Substrate q cal = kj/mol (with island size) 2D Cu Platelets Large islands: H ad = 310 kj/mol = 2 BE(Cu-Cu) + BE(Cu-MgO) BE(Cu-MgO) = ~78 kj/mol from E adh 192 µj/cm 2 Top Layer: BE(Cu-Cu(100)) 4 x 56.2 kj/mol + 24 kj/mol = 249 kj/mol BE(Cu-Cu(100)) = 4 x 56.2 kj/mol = 225 kj/mol (bulk) MgO(100) Substrate q cal = H sub = 6 x 56.2 kj/mol = 337 kj/mol 3D Cu Islands Summary of average bonding energetics for Cu to MgO(100) Extracted from the calorimetry measurements. Assumes pairwise bond-additivity for Cu-Cu bonds (i.e., BE(Cu-Cu) = H sub / 6= 56.2 kj/mol).

14 2D M-MgO(100) Bond Energy (kj / mol M) Pb Correlation of 2D M-MgO(100) Bond Energy with Sublimation Enthalpy of Metal H sublim (kj / mol M) Ag Cu Campbell et al., JACS 124 (2002) Suggests that covalent metal-mg bonding dominates the interaction for 2D particles. Probably due to very strong bonding at defects to coordinatively unsaturated Mg atoms.

15 Thin single crystal sample Pyroelectric Ribbon for Temperature Rise Detection Pulsed Metal Atom Source V Moved into contact with back of thin sample Metal Nanoparticle Energy of added atom / (kj/mol) Metal atom energy versus size of particle it is in. B u lk E n e rg y Metal particle radius / nanom eters Campbell, Parker & Starr, Science 298 (2002) 811.

16 Catalyst Sintering Nanoparticles are unstable wrt larger particles They often sinter (grow in size, decrease in number) during use. Big problem is catalysis. Slows rate of new catalyst development. We study mechanisms of sintering with nc-afm and STM and are developing accurate kinetic models for sintering, in hopes this will speed development of new catalysts for a cleaner environment.

17 Studies of Pd nanoparticles on oxides Oxide-supported metal particle catalysts reduce greenhouse gas emissions. For example, Pd on alumina catalyzes the low-temperature combustion of methane, thus reducing dramatically NO x emissions. Commercialization limited by Pd sintering problems. Nano-scale metal particle catalysts can exhibit greater efficiency than larger particles because of: higher surface area unsaturated bond coordination. Explore Pd sintering and particle size effects on the dissociative adsorption of Methane and other hydrocarbons. Dissociation may be more facile on small particles.

18 Non-Contact Atomic Force Microscopy (NC-AFM) of reconstructed α-al 2 O 3 (0001) surface 1300 nm O layer spacing is about 0.2 nm in α-al 2 O nm Z scale 2.3 nm Bulk structure from E.A. Soares, et al., Phys. Rev. B 65, (2002).

19 Terraces on clean α-al 2 O 3 (0001) 1500nm x 1120nm NC-AFM image of the clean unreconstructed alumina surface. Surface prepared by annealing in air, Ar + sputtering and annealing in vacuum at 1070K. height (nm) position (µm) Terraces are ~300 nm wide and 0.2 or 0.4 nm tall, probably corresponding to the O layer separation in the unit cell. 0.2nm

20 0.4 ML Pd / clean α-al 2 O 3 (0001) height (nm) nm x 102 nm position (nm) Clean alumina surface Peak -to- peak roughness < 0.10 nm. *Surf. Sci. 323 (1995) nm x 102 nm height (nm)0.14 position (nm) 0.4 ML Pd / alumina, at 300 K The Pd must be in the form of 2D islands (consistent with Cordatos et al.*), showing corrugation < 0.12 nm.

21 0.4 ML Pd / clean α-al 2 O 3 (0001) height (nm) nm x 100 nm m10x vs m10hori m10x vs m10vert m20x vs m20hori m20x vs m20vert position (nm) 0.4 ML Pd, annealed to 600 K Clusters are nm high and 3-6 nm in diameter. height (nm) nm x 100 nm position (nm) 0.4 ML Pd, annealed to 900 K. Clusters are nm high and 8-10 nm in diameter. There is also a higher density of clusters.

22 0.1 ML Pd / α-al 2 O 3 (0001) 219 nm x 219 nm 219 nm x 219 nm 200 nm x 200 nm height (nm) position (nm) Clean alumina surface Step is 0.4 nm high and terraces have a p-to-p corrugation = 0.21 nm. height (nm) position (nm) 0.1 ML Pd / Al 2 O 3, at RT P-to-p corrugation on terraces = 0.21 nm. height (nm) position (nm) 0.1 ML Pd, annealed to 670K The clusters are nm tall and ~10 nm wide. The clusters may appear wider than they actually are due to tip convolution.

23 Pd Clusters seen at Room Temperature by AFM 80 nm 80 nm Coverage of 1.1 ML estimated from AFM image by Pang et al., Surf. Sci. 460 (2000), L510. Also NC-AFM on the reconstructed alumina surface (single crystal). Particles are 4-6 nm in diameter and nm in height. In our case (bottom), we know Pd coverage is only 0.2 ML.

24 After deposition of 0.8 ML Pd and heating to 870 K 244 nm ¾ of Pd particles are at edges of depressions. Depressions are 0.4 nm deep. Z scale = 4 nm 495 nm

25 Low-energy He + Ion Scattering Spectroscopy (LEIS) Probes elemental composition of top 0.3 Å LEIS Intesity (arb. units) O B e fo re P d d o se After 1.6 M L Pd dose Al Pd Kinetic Energy (ev) Provides rapid, direct, quantitative measure of area fraction covered by metal nanoparticles.

26 Sintering Kinetics of Metal Particles on Oxide Supports Wynblatt and Gjostein Model (interface control limit) in J. O. McCaldin and G. A. Somorjai (Eds.): Progress in Solid State Chemistry, Vol , p. 21; and Acta Metallurgica 29 (1981) 221. dr/dt = (K/R) e -Etot/kT [e {µ(r*)-µ( )}/kt - e {µ(r)-µ( )}/kt ] R = particle s radius at time t R * = 1 / (average of 1/R for all particles) K = depends on prefactor (ν) and contact angle. E tot = E oxide 2Dvap + E diff µ(r) = chemical potential of a metal atom in a particle of radius R Note: µ(r)-µ( ) = 2γΩ/R where γ = surface free energy of solid metal Ω = atomic volume of solid metal = molar volume / N A Problems: 1. Surface energy assumed constant, independent of radius. 2. Exponentials simplified Problem: to 1 st term in Taylor series expansion. Corrections to Model: Replace chemical potential with measured energy vs. size. This removes assumption of constant surface energy. Extend Pb measurements to other metals assuming µ(n) scales with H sub. (N = # atoms in particle) Keep exponential. All previous researchers assumed Gibbs-Thompsom eqn. w/ constant γ. The next page shows this clearly gives a HUGE error. Instead, we use our calorimetric measurements of metal adsorption energies to directly measure the dependence of the chemical potential (µ, which is really a Gibb s free energy) upon R/

27 Pb on MgO(100) 220 Gibbs-Thompson µ(r) - µ( ) = 2γΩ/R γ = 59 µj/cm Constant γ Model ( γ = 59 µ J/m ol) H sub = kj/m ol Heat of Adsorption (kj/mol) Calorimetry Data Metal Island Radius (nm) Campbell, Parker & Starr, Science 298 (2002) 811.

28 Simulation of sintering kinetics vs. experiment. dr/dt = (K/R) e -Etot/kT [e {µ(r*)-µ( )}/kt -e {µ(r)-µ( )}/kt ] Au Area Fraction (ML) Initial Au / TiO 2 (110) β = 1 K/s Experiment: TP-LEIS (He + ) Simulation w/ Gibbs-Thompson Eqn. Simulation: µ(r) from calorimetry, assuming α H sub (Initial particle size dist. based on STM from Lai and Goodman (2000).) Temperature (K) Parameters with new model physically reasonable (for first time): E oxide ads -E oxide diff = 41 kj/mol. Campbell, Parker & Starr, Science 298 (2002) 811.

29 Traditional Catalyst: Sinter-resistant Nanostructures: RIGHT: Schematic representations of: (a) the traditional 2-phase catalyst consisting of oxidesupported metal nanoparticles, and two new 3-phase nanostructures: (b) metal nanoparticles supported on a surface patterned in domains of two oxides which bind the metal weakly and strongly, respectively, and (c) metal nanoparticles, decorated on their edges with an additive, supported on an oxide. LEFT: Diagrams showing the atomic structure at the edge of one of the metal nanoparticles in each structure, and the energy barriers associated with moving the gray metal atom off the cluster edge and across the oxide surface along the path shown.

30 David Starr and Linda Jung Steve Parker and Jane Larsen

31 Studies of Pd nanoparticles on oxides Oxide-supported metal particle catalysts reduce greenhouse gas emissions. For example, Pd on alumina catalyzes the low-temperature combustion of methane, thus reducing dramatically NO x emissions. Commercialization limited by Pd sintering problems. Nano-scale metal particle catalysts can exhibit greater efficiency than larger particles because of: higher surface area unsaturated bond coordination. Explore Pd sintering and particle size effects on the dissociative adsorption of Methane and other hydrocarbons. Dissociation may be more facile on small particles.

32 C Titration by O 2 beam vs. increasing CH 4 exposure CO 2 desorption rate CO2 CH 4 Pd MgO Pd CO desorption rate CO O 2 CO 2 CO C C C C C C C Pd Pd MgO Time (s)

33 Normalized C coverage Effect of Pd coverage (particle size) on CH 4 dissociation rate Steve Tait, Z. Donalek, B. D. Kay (at PNNL) 0.5 ML Pd / MgO(100) 0.9 ML Pd / MgO(100) 100 ML Pd film CH4 exposure (MLE)

34 Steven Hongbo Jennifer Hyeran Carsten Charlie Louis Ann Lien David Mack Henry Steve

35 GREEN CHEMISTRY through NanoSTRUCTURED MATERIALS Catalysts for pollutant cleanup, chemical production, fuel conversion: Tunable particle diameter, tunable pore size. Activity, selectivity: tunable with dimensions. Homogeneous structure: key to selectivity. Higher selectivity (less waste, less pollution). Lower energy consumption. Understanding provided by nanoscience: faster development of better catalysts. Sorbents, membranes, etc. for same.

36 Acknowledgements L. Ngo, A. W. Grant, C. Stegelmann, L. Xu, J. Larsen, S. L. Tait, H. Ihm, H. Ajo, V. A. Bondzie, D. E. Starr, S. Lehto, C. A. Perez (COPPE), M. Schmal (COPPE), B. D. Kay (PNNL) and Z. Dohnalek (PNNL) U. S. Department of Energy Division of Chemical Sciences Univ. of Washington Center for Nanotechnology and its NSF-IGERT Program Univ. Wash. / PNNL Joint Institute in Nanoscience