Nanostructured Catalysts. Abhaya K. Datye University of New Mexico

Transcription

1 Nanostructured Catalysts Abhaya K. Datye University of New Mexico

2 Issues Control of surface composition Facile synthesis via self assembly Aggregation of nanoparticles

3 Control of Surface Composition and Structure in Nanoparticles Selective catalysts often involve more than one element Thermodynamics, preparation variables, often dictate the surface composition and structure How do we generate tailored surface structures

4 Restructuring of Pd-Ag Catalysts During Selective Hydrogenation of Trace Acetylene in Ethylene K. Lester, Y. Jin, H. Zea and A. K. Datye University of New Mexico, Center for Microengineered Materials and a Department of Chemical & Nuclear Engineering, Albuquerque, NM 87131, USA E. G. Rightor 1, R. J. Gulotty 1, J. J. Maj 1, J. Blackson 1, M. Holbrook 2 and C. Michael Smith 3 The Dow Chemical Company, 1 Midland, MI, 48674, 2 Plaquemine, LA 77565, 3 Freeport, TX 77541,, USA. Financial support provided by the U. S. DOE, Office of Basic Energy Sciences, grant DE-FG03-98ER14917 and by the Dow Chemical Company

5 Restructuring of Pd-Ag Catalysts During Selective Hydrogenation of Trace Acetylene in Ethylene Industrial feedstock for the production of ethylene polymers must contain no more than 5-10 ppm of acetylene. Selective Hydrogenation of acetylene is used to remove trace acetylene C 2 H 2 + H 2 C 2 H 4 + H 2 C 2 H 6 Ethylene selectivity is a key objective in this process. Catalysts are subject to thermal runaway due to the exothermic reaction

6 Operating Conditions Our reaction conditions correspond to the front end hydrogenation, where acetylene is present with a large excess of ethylene and also an excess of hydrogen and some CO. Feed:30% C 2 H 4, 0.4% C 2 H 2, 0.1% CO, 16% H 2 and balance N 2.

7 Hydrocarbon byproduct formation is suppressed on Pd-Ag after HTR Selectivity to Oligomers vs Delta Temperature Pd/Al 2 O 3 No pretreatment Selectivity to Oligomers Pd+Ag/Al 2 O C Pretreatment Delta Temperature

8 High Temperature Reduction Causes a Drop in Activation Energy for Ethylene Hydrogenation on Bimetallic Pd-Ag catalysts C 2 H 2 C 2 H 4 C 2 H 6 Selectivity = moles ethylene produced moles acetylene reacted 0.5Pd-0.5Ag / SiO 2 Catalysts - Selectivity Vs Delta Temperature. Arrhenius plot for 0.5 Pd Ag/SiO2 catalysts Ethylene Selectivity Delta Temperature (oc) Reduced at 500 C Reduced at 100 C ln(ethene formed) Eactivation (kcal/mol) = Eactivation (kcal/mol) = /T (1/K) Reduced at 500 C Reduced at 100 C T = reaction temperature clean up temperature Clean up temperature is the temperature at which 99% of acetylene conversion is obtained

9 Ethylene Hydrogenation Is A Structure-Insensitive Reaction Why should the activation energy for ethylene hydrogenation be affected by pretreatment?

10 Effect of CO adsorption on Activation Energy for Ethylene Hydrogenation If the surface is covered by CO, the activation energy for ethylene reaction is simply the heat of desorption of CO Therefore, changes in the heat of desorption of CO can change the activation energy for ethylene hydrogenation

11 On Pd/SiO 2, CO is adsorbed mainly in a bridged mode There is no effect of pretreatment Bridge Bridge Linear Linear Reduced at 70 C Reduced at 400 C

12 On Pd-Ag/SiO 2, we see more linearly bound CO than bridged. High temperature reduction further affects the relative concentrations of linear vs bridged CO Linear Bridge Linear Bridge Reduced at 70 C Reduced at 400 C

13 Effect of Reduction Temperature Pd-Ag alloy, with some phase segregation Ag redistributes causing a breakup of the Pd ensembles Low temperature reduction High Temperature reduction

14 Pd/ SiO 2 We see no effect of pretreatment on ethylene hydrogenation activation energy Pretreatment Activation Energy as a function 100 C of Pretreatment 500 C Activation Energy (Apparent) kcal/mol The apparent activation energy for ethylene hydrogenation on Pd is consistent with the heat of adsorption of CO. From the literature, the heat of adsorption for bridged CO ranges from kcal/mol depending on coverage.

15 Bridged CO is more strongly bound than linearly bonded CO Heats of Adsortion of the Adsorbed CO Species on the Various Pd-Cointaining Solids at Low (E0) and High (E1) Coverage [1] Linear CO species Bridged CO Species Sample E0 (kcal/mol) E1(kcal/mol) E0 (kcal/mol) E1(kcal/mol) Pd (Cl-free)/Al2O Pd (Cl)/Al2O Pd (Cl-f)/CeO2/Al2O Pd (Cl)/CeO2/Al2O Pd (Cl)/La2O3/CeO2/Al2O (Cl-f): Chlorine free solid (Cl): Chlorine containing solid [1] Dulaurent O, Chandes K, Bouly C and Bianchi D, Journal of Catalysis, v 192(#2), 2000

16 Arrhenius plot for Ethylene Hydrogenation on 0.5 Pd Ag/SiO2 catalysts -13 Eactivation (kcal/mol) = ln(ethane formed) Eactivation (kcal/mol) = /T (1/K) Reduced at 500 C Reduced at 100 C

17 Schematic of Restructuring Phenomena in Pd-Ag High Temperature Reduction High Temp Oxidation PdO Ag Pd-Ag alloy Enrichment of Ag on Pd surface Low Temp Oxidation Pd Ag 2 O

18 Summary High temperature pretreatments cause restructuring of Pd and Ag Reducing the number of Pd nearest neighbors affects selectivity to oligomer formation By modifying the adsorption of coadsorbed CO, we can control the activation energy for ethylene hydrogenation and modify the selectivity for the reaction

19 Aerosol Synthesis of Nanostructured Catalysts Mangesh Bore, Hien Pham, Timothy Ward, C. J. Brinker, Abhaya Datye Financial Support provided by NSF NIRT, Center for Ceramic and Composite Materials and by the Materials Corridor Council

20 Autoclave Route Reaction Mixture Autoclave 150 o C 48 hours Filtration Calcination Liquid-Crystal Template Mechanism Proposed by C. T. Kresge et al., Nature (1992) J. S. Beck et al., J. A. C. S. (1992)

21 MCM-41 Irregular shapes

22 Aerosol Route Calcination Evaporation Induced Self Assembly (EISA) Proposed by Jeffrey Brinker et. al., Nature (1999) Evaporation of solvent leads to ordering of surfactant structures Condensation of silica follows the formation of templated structures to lock in the structure

23 Control of Particle Structure Y. Lu, H. Fan, A. Stump, T.L. Ward, T. Rieker, C.J. Brinker, Nature 398 (1999) 223 Hexagonal nanostructure: interconnected hexagonally packed spherical pores, 1200 m 2 /g, d=3.2 nm (5 wt% CTAB) cubic nanostructure: interconnected pores arranged in simple cubic lattice (4.2 wt% B56) lamellar onion-skin structure: concentric shells of silica separated by pore volume, 478 m 2 /g, d=9.2 nm (5wt% P123)

24 Comparison Aerosol Synthesis Continuous process Reaction time seconds Spherical particles 3-D interconnected pore structure (local order is hexagonal) Autoclave Synthesis Batch process Reaction time hours Irregular shapes Most common is the 2-D structure

25 TEM Regular shapes Particle consists of small ordered domains of pores

26 After Hydrothermal Stability Test at 750 C 10% water vapor, 2 hours SiO 2 Si/Al 20 Aluminum incorporation improves the hydrothermal stability of mesoporous silica particles.

27 Hydrothermal Stability Test (batch vs. aerosol route) 10% water vapor, 2 hours Si/Al molar ratio 20 Surface Area (sq m/gm) Batch Si-Al Aerosol Si-Al 400 Davisil silica gel Aerosol Silica Initial Temperature (C) Batch Silica

28 TEM/STEM images of Au/NH 2 -MCM-41 The average Au nanoparticle diameter is small (~1 nm), and the nanoparticles are dispersed inside the pores. 3-aminopropyltrimethoxysilane is used as the amine source

29 TEM Images of Ordered Nanocrystal/Silica Nanostructures [100] Before calcination A 20 nm B C Courtesy of Hongyou Fan, Jeff Brinker Sandia National Laboratories

30 Diffusion of Three-Dimensional Metal Particles on an Oxide Substrate: Implications for the Sintering of Heterogeneous Catalysts Lani Miyoshi Sanders Abhaya K. Datye Univ. of New Mexico, Albuquerque, NM Brian Swartzentruber Sandia National Labs, Albuquerque, NM

31 Experimental Approach: Atom-Tracking Scanning Tunneling Microscopy of Pd/TiO (110) 2

32 Conventional STM Each image takes several seconds, missing many rotation events time resolution 1000x Si-Ge Ad- Dimer on Si(001) Atom Tracking Tip STM Developed by Brian Swartzentruber, Sandia Labs y g t1 t2 x

33 TiO 2 (110) Surface 100x100Å 2 3Åx6.5Å Unit cell From: M.J.J.Jak, Ph.D. Dissertation, Nov

34 Depositing Pd 300x300Å 2 ast 4 W, 2 s 300x300Å 2 Slow 3 W, 4 min

35 Diffusion Characteristics Diffusion is essentially confined to the [001] direction Presence of small, mobile particles rapidly decays due to pinning and growth Step decoration is prevalent only on steps perpendicular to [001] 400x600Å 2 Diffusing particles hop discretely with length of underlying unit cell of substrate

36 Atom-Tracking of Pd Particle Diffusion [001] Y (Angstrom) X (Angstrom) x100Å Time (s)

37 Scaling Analysis for Pd Particle Diffusion 1E-14 Diffusion Coefficient (cm^2/s) 1E-15 1E-16 1E-17 n=0.86±0.09 n=1.06±0.10 n=1.07± C 36 C 25 C Particle Size (# of Atoms)

38 3-D D Monte Carlo model gives insights into decreased motion of larger particles disordered surfaces high surface free energy hopping d -1 very small particles shift in diffusion mechanism grow shift in scaling law larger particles periphery diffusion d -7 faceting 300x300Å 2

39