Abhaya Datye - Department of Chemical Engineering

Nanostructured CatalystsAbhaya K. DatyeUniversity of New Mexico

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

Control of Surface Compositionand Structure in Nanoparticles• Selective catalysts often involve more thanone element• Thermodynamics, preparation variables,often dictate the surface composition andstructure• How do we generate tailored surfacestructures

Restructuring of Pd-Ag CatalystsDuring Selective Hydrogenationof Trace Acetylene in EthyleneK. Lester, Y. Jin, H. Zea and A. K. DatyeUniversity of New Mexico, Center for Microengineered Materials and aDepartment of Chemical & Nuclear Engineering, Albuquerque, NM 87131,USAE. 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, 48674,2Plaquemine, LA 77565, 3 Freeport, TX 77541, , USA.Financial support provided by the U. S. DOE, Officeof Basic Energy Sciences, grant DE-FG03-98ER14917 and by the Dow Chemical Company

Operating Conditions• Our reaction conditions correspond to the‘front end’ hydrogenation, where acetylene ispresent with a large excess of ethylene andalso 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 .

Hydrocarbon byproduct formation issuppressed on Pd-Ag after HTRSelectivity to Oligomers vs DeltaTemperature0.350.3Pd/Al 2O 3No pretreatmentSelectivity toOligomers0.250.20.150.1Pd+Ag/Al 2O 3500 C Pretreatment0.050-25 -20 -15 -10 -5 0 5 10 15 20 25Delta Temperature

High Temperature Reduction Causes a Drop inActivation Energy for Ethylene Hydrogenationon Bimetallic Pd-Ag catalystsC 2H 2 C 2H 4 C 2H 6Selectivity = moles ethylene producedmoles acetylene reacted0.5Pd-0.5Ag / SiO 2 Catalysts - Selectivity VsDelta Temperature.Arrhenius plot for 0.5 Pd - 0.5 Ag/SiO2catalystsEthylene Selectivity10.50-0.5-1-1.5-2-2.50 10 20 30 40Delta Temperature (oC)Reduced at 500 CReduced at 100 Cln(Ethene formed)-13-15-17-19Eactivation (kcal/mol) = 20.5 + 0.4Eactivation (kcal/mol) = 13.5 + 0.30.0024 0.0026 0.00281/T (1/K)Reduced at 500 C Reduced at 100 C∆T = reaction temperature – clean up temperatureClean up temperature is the temperature at which 99% of acetylene conversion isobtained

Ethylene Hydrogenation Is AStructure-InsensitiveReactionWhy should the activation energyfor ethylene hydrogenation beaffected by pretreatment?

Effect of CO adsorption onActivation Energy for EthyleneHydrogenationIf the surface is covered by CO, the activationenergy for ethylene reaction is simply theheat of desorption of COTherefore, changes in the heat of desorptionof CO can change the activation energy forethylene hydrogenation

On Pd/SiO 2 , CO isadsorbed mainly in abridged modeThere is no effect ofpretreatmentBridgeBridgeLinearLinearReduced at 70 CReduced at 400 C

On Pd-Ag/SiO2 , we see morelinearly bound CO thanbridged. High temperaturereduction further affects therelative concentrations of linearvs bridged COLinearBridgeLinearBridgeReduced at 70 CReduced at 400 C

Effect of Reduction TemperaturePd-Ag alloy, with some phasesegregationAg redistributes causing abreakup of the Pd ensemblesLow temperature reductionHigh Temperaturereduction

Pd/ SiO 2We see no effect of pretreatment onethylene hydrogenation activation energyPretreatmentActivation Energy as a function 100 C of Pretreatment500 CActivation Energy(Apparent) kcal/mol28 27The apparent activation energy for ethylene hydrogenation on Pdis consistent with the heat of adsorption of CO. From theliterature, the heat of adsorption for bridged CO ranges from 22-40kcal/mol depending on coverage.

Bridged CO is more stronglybound than linearly bonded COHeats of Adsortion of the Adsorbed CO Species on the VariousPd-Cointaining Solids at Low (E0) and High (E1) Coverage [1]Linear CO species Bridged CO SpeciesSample E0 (kcal/mol) E1(kcal/mol) E0 (kcal/mol) E1(kcal/mol)Pd (Cl-free)/Al2O3 22 13 40 22Pd (Cl)/Al2O3 22 13 40 18Pd (Cl-f)/CeO2/Al2O3 22 13 40 22Pd (Cl)/CeO2/Al2O3 22 13 40 16Pd (Cl)/La2O3/CeO2/Al2O3 22 13 40 25(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

Arrhenius plot for Ethylene Hydrogenation on 0.5 Pd - 0.5Ag/SiO2 catalysts-13Eactivation (kcal/mol) = 20.5 + 0.4ln(Ethane formed)-15-17Eactivation (kcal/mol) = 13.5 + 0.3-190.0024 0.0026 0.00281/T (1/K)Reduced at 500 CReduced at 100 C

Schematic of RestructuringPhenomena in Pd-AgHigh Temperature ReductionHigh Temp OxidationPdOAgPd-Ag alloyEnrichment of Ag on Pd surfaceLow Temp OxidationPdAg 2O

Summary• High temperature pretreatments causerestructuring of Pd and Ag• Reducing the number of Pd nearestneighbors affects selectivity to oligomerformation• By modifying the adsorption of coadsorbedCO, we can control the activation energyfor ethylene hydrogenation and modify theselectivity for the reaction

Aerosol Synthesis of Nanostructured CatalystsMangesh Bore, Hien Pham, Timothy Ward,C. J. Brinker, Abhaya DatyeFinancial Support provided by NSF – NIRT, Centerfor Ceramic and Composite Materials and by theMaterials Corridor Council

Autoclave RouteReaction MixtureAutoclave150 o C48 hoursFiltrationCalcination• Liquid-Crystal Template Mechanism– Proposed by C. T. Kresge et al., Nature (1992)J. S. Beck et al., J. A. C. S. (1992)

MCM-41Irregular shapes

Aerosol RouteCalcination• Evaporation Induced Self Assembly (EISA)– Proposed by Jeffrey Brinker et. al., Nature (1999)– Evaporation of solvent leads to ordering of surfactantstructures– Condensation of silica follows the formation of templatedstructures to lock in the structure

Control of Particle StructureY. Lu, H. Fan, A. Stump, T.L. Ward, T. Rieker, C.J. Brinker,Nature 398 (1999) 223Hexagonal nanostructure:interconnected hexagonallypacked spherical pores,1200 m 2 /g, d=3.2 nm (5 wt%CTAB)cubic nanostructure:interconnected poresarranged in simple cubiclattice (4.2 wt% B56)lamellar “onion-skin”structure: concentricshells of silica separatedby pore volume, 478m 2 /g, d=9.2 nm (5wt%P123)

ComparisonAerosol Synthesis• Continuous process• Reaction time seconds• Spherical particles• 3-D interconnected porestructure (local order ishexagonal)Autoclave Synthesis• Batch process• Reaction time hours• Irregular shapes• Most common is the 2-Dstructure

TEMRegular shapesParticle consists of small ordered domains of pores

After Hydrothermal Stability Test at 750°C10% water vapor, 2 hoursSiO 2Si/Al 20Aluminum incorporation improves the hydrothermal stability of mesoporous silicaparticles.

Hydrothermal Stability Test (batch vs. aerosol route)10% water vapor, 2 hours16001400Si/Al molar ratio 20Surface Area (sq m/gm)12001000800600Batch Si-AlAerosol Si-Al400Davisil silica gelAerosol Silica2000Initial 500 550 600 650 700 750Temperature (C)Batch Silica

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

TEM Images of Ordered Nanocrystal/Silica Nanostructures[100]Before calcinationA20 nmBCCourtesy of Hongyou Fan, Jeff BrinkerSandiaNationalLaboratories

Diffusion of Three-Dimensional Metal Particleson an Oxide Substrate: Implications for theSintering of Heterogeneous CatalystsLani Miyoshi SandersAbhaya K. DatyeUniv. of New Mexico, Albuquerque, NMBrian SwartzentruberSandia National Labs, Albuquerque, NM

Experimental Approach:Atom-Tracking ScanningTunneling Microscopy ofPd/TiO (110)2

Conventional STMEach image takes severalseconds, missing many rotationevents…time resolution 1000xSi-Ge Ad-Dimer onSi(001)Atom TrackingTipSTMDeveloped by Brian Swartzentruber, Sandia Labsygt1t2x

TiO 2 (110) Surface100x100Å 23Åx6.5ÅUnit cellFrom: M.J.J.Jak, Ph.D. Dissertation, Nov.2000.

DepositingPd300x300Å 2 300x300Å 2ast deposit @ 4 W, 2 sSlow deposit @ 3 W, 4 min

DiffusionCharacteristicsDiffusion isessentially confinedto the [001] directionPresence ofsmall, mobileparticlesrapidlydecays due topinning andgrowthStepdecoration isprevalent onlyon stepsperpendicularto [001]400x600Å 2Diffusing particles hop discretely with lengthof underlying unit cell of substrate

Atom-Tracking of PdParticle Diffusion[001]Y (Angstrom)165160155150145140135130145 155X (Angstrom)165160155150145140135130100x100Å 20 10 20 30 40 50Time (s)

Scaling Analysis for Pd ParticleDiffusion1E-14Diffusion Coefficient (cm^2/s)1E-151E-161E-17n=0.86±0.09n=1.06±0.10n=1.07±0.1042°C36°C25°C1 10 100Particle Size (# of Atoms)

3-D D Monte Carlo model gives insights intodecreased motion of larger particlesdisordered surfaceshigh surface free energyhoppingd -1very small particlesshift indiffusionmechanismgrowshift inscaling lawlarger particlesperiphery diffusiond -7faceting300x300Å 2