www.sciencemag.org/cgi/content/full/339/6127/1590/DC1

Supplementary Material for

Tuning Selectivity in Propylene Epoxidation by Plasmon Mediated Photo-Switching of Cu Oxidation State

Andiappan Marimuthu, Jianwen Zhang, Suljo Linic*

*Corresponding author. E-mail: linic@umich.edu

Published 29 March 2013, Science 339, 1590 (2013)

DOI: 10.1126/science.1231631

This PDF file includes:

Materials and Methods

Figs. S1 to S6

References (30, 31)

Supplementary Material

Tuning selectivity in propylene epoxidation by plasmon mediated photo-switching of Cu oxidation state

Andiappan Marimuthu, Jianwen Zhang, Suljo Linic*

Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA.

*Correspondence to: linic@umich.edu

This file includes:

Materials and Methods

Figs. S1-S6

References (30,31)

Materials and Methods

Section S1. Preparation of catalyst: Copper nanoparticles catalysts were prepared using (water

in oil) micro emulsion synthesis method reported previously (30). n-heptane (99% purity, Sigma-

Aldrich Cat. No. 246654), polyethylene glycol-dodecyl ether (average Mn ~362, Sigma-Aldrich

Cat. No. 235989), copper nitrate (99.999%, Sigma-Aldrich Cat. No. 229636) and hydrazine

(98% purity, Sigma-Aldrich Cat. No. 215155) were used as continuous oil phase, surfactant,

copper precursor and reducing agent, respectively. Water to surfactant (molar) ratio of 30 and

reducing agent to precursor ratio of 10 were used for the synthesis. Briefly, an aqueous solution

of copper precursor (0.1 M Cu(NO3)2) was added to the mixture of n-heptane and surfactant

(16.54 wt%) under inert atmosphere (argon). After the mixture was equilibrated, an aqueous

solution of reducing agent (1 M hydrazine) was added drop by drop to the mixture and the

microemulsion was kept under continuous stirring in inert atmosphere (argon) at room

temperature for the complete reduction of copper precursor. A known amount of support

material, SiO2, (Surface area ~80-100 m2/g, Alfa Aesar, Product No. 42737) was then added

under stirring to the microemulsion containing copper nanoparticles to prepare catalyst with final

composition of ~2 wt % Cu/SiO2. Acetone was then added to the mixture under stirring to break

the microemulsion and remove the surfactant. The catalyst was further washed with water and

ethanol by centrifugation and dried in argon at room temperature. The scanning electron

microscope (SEM) images of the copper nanoparticles synthesized showed that the particles are

quasi-spherical in shape with average size of 41 ± 9 nm. A representative SEM image of the

copper nanoparticles is shown in Figure S1. Micrographs were obtained using an FEI Nova 200

Nanolab and the accelerating voltage used for the imaging was 10 kV.

Section S2. Reactor studies: The reactions were performed in a vertically oriented packed bed

reactor (Harrick High Temperature Reaction Chamber) that allowed both temperature control

through electrical heating at the bottom and light illumination through 1cm2 silica window at the

top of the reactor (8). The light source (Dolan-Jenner Fiber-Lite 180 Model illuminator with

single gooseneck fiber optics) used for the photothermal studies (light on) was a broadband

visible light source with a maximum spectral intensity at ~580 nm (Figure S2). The source

intensity was varied by controlling the source power and the total light intensity was measured

using the combination of UV intensity meter (OAI Instruments) and CCD camera (Avantes

AvaSpec-2048). For the wavelength dependent phototothermal experiments, optical filters

(Edmund Optics) were used to control the wavelength of light reaching the catalyst from the

light source and the total intensity of light reaching the catalyst surface was kept constant (~ 550

mW/cm2) for all experiments by adjusting the power to the light source. The catalyst (packed)

bed was prepared by loading 10 mg of inert silica beads (Silica gel, Sigma-Aldrich Cat. No.

214418) to the bottom of the reactor and 12 mg of 2 wt% Cu/SiO2 catalysts on top of the layers

of silica beads. For all reactions reported here, the catalysts were pre-reduced at 523K for 1 hr in

20% hydrogen (remaining helium) at a total flow rate of 100 cm3/min before starting the

reactions. For propylene epoxidation reaction, the reactants with a composition of 50%

propylene and 50% oxygen at a total flow rate of 100 cm3/min were delivered from the top of the

catalyst bed under specified reaction conditions. The reactants and products were analyzed

online using a gas chromatograph (Varian CP 3800) equipped with thermal conductivity and

flame ionization detectors.

The rate and selectivity results reported here for thermal (light off) and photothermal (light on)

conditions were measured in a differential reactor set-up in the regime limited by the reaction

kinetics under steady state reaction conditions. Propylene oxide (PO), acrolein (Acr) and carbon

dioxide (CO2) were observed as the principal products. The reaction rates were calculated using

the formula, (CPO+CAcr+CCO2/3)/W, where, CPO, CAcr, CCO2 and W denote the measured

concentration of the products- PO, acrolein and CO2, and the weight of the catalyst, respectively.

The PO, acrolein and CO2 selectivity were calculated using the formulae-

CPO/(CPO+CAcr+CCO2/3), CAcr/(CPO+CAcr+CCO2/3) and (CCO2/3)/(CPO+CAcr+CCO2/3), respectively.

The reaction rate and selectivity data shown in the figures represent the average values of the

results obtained from multiple independent experiments and the error bars in the data represent

the standard deviations. The selectivity to all carbon-based reaction products at T = 473 K as a

function of light intensity is shown in figure S3. The selectivity of all carbon-based products as a

function of temperature at light intensity of 550 mW/cm2 is shown in figure S4. The selectivity

to all carbon-based products as a function of temperature for non-illuminated catalysts is shown

in figure S5.

Section S3. Characterization of catalyst: UV-Vis extinction spectra of the catalysts were

measured using Thermo Scientific Evolution 300 UV-Vis Spectrophotometer with a Harrick

Praying Mantis Diffuse Reflection Accessory. Harrick High Temperature Reaction Chamber

used to carry out reactions under thermal and photothermal conditions is equipped with silica

windows which allow recording of the UV-Vis spectra of the catalyst under in situ conditions.

All in situ UV-Vis measurements were acquired under inert atmosphere (100 cm3/min of helium)

at specified temperature. For in situ UV-Vis measurements under photothermal conditions (light

on), the experiments were first carried out as described in the previous section. Before measuring

the UV-Vis spectrum, the flow of reactants (100 cm3/min) was exchanged for helium (100

cm3/min) for ~15 minutes. The light source used for the photothermal experiments was then

turned off and the UV-Vis spectrum was measured in inert atmosphere (100 cm3/min of helium).

The identical procedure was followed for measuring the in situ UV-Vis spectra of the catalysts

under thermal conditions.

X-ray diffraction (XRD) patterns of the used catalysts were acquired using Rigaku Miniflex

DMAX-B rotating anode X-ray diffractometer with Cu-Kα radiation source operated at 30 kV

and 15 mA.

Section S4. Finite-difference Time-domain (FDTD) simulation: Finite-difference time-

domain (FDTD) optical simulations were performed to simulate the extinction cross section of

the copper nanoparticle as a function of wavelength of incident light. Drude-Lorentz model was

used to represent the optical properties. The empirical optical constants of copper metal were

taken from Palik (31). The periodic boundary conditions were used in x- and y-directions, while

the perfectly matched layer (PML) boundary condition was used in the z-direction. The

extinction cross sections as a function of wavelengths were calculated using the total-

field/scattered-field (TFSF) formalism. The incident light source used for the simulation was

400-1000 nm Gaussian source.

Fig. S1. Top: SEM image of copper nanoparticles dispersed on SiO2 support. Smaller particles are Cu. Bottom: SEM image of copper nanoparticles deposited on a silicon wafer.

Fig. S2. Spectrum of the light source used in the photothermal studies.

0

0.2

0.4

0.6

0.8

1

400 500 600 700 800 900 1000

Rela

tive

Inte

nsity

(a.u

)

Wavelength (nm)

Light Source

Fig. S3. Selectivity to PO, acrolein and CO2 under photothermal conditions at 473K as a function of light intensity. The selectivity data shown in the figure represent the average values of the data obtained from five independent experiments and the error bars represent the standard deviations.

0

10

20

30

40

50

60

70

100 300 500 700 900

Sele

ctiv

ity (%

)

Intensity (mW/cm2)

CO2AcroleinPO

Fig. S4. Selectivity to PO, acrolein and CO2 for photothermal (light on) process as a function of temperature. The light intensity used for the photothermal studies was 550 mW/cm2. The selectivity data shown in the figure represent the average values of the data obtained from three independent experiments and the error bars represent the standard deviations.

0

10

20

30

40

50

60

70

470 490 510 530 550

Sele

ctiv

ity (%

)

Temperature (K)

CO2

Acrolein

PO

Fig. S5. Selectivity to PO, acrolein and CO2 under thermal conditions (light off) as a function of temperature. The selectivity data shown in the figure represent the average values of the data obtained from three independent experiments and the error bars represent the standard deviations.

0

10

20

30

40

50

60

70

520 540 560 580

Sele

ctiv

ity (%

)

Temperature (K)

CO2AcroleinPO

Fig. S6. Extinction spectrum of copper spherical nanoparticle (41 nm) calculated from FDTD simulation.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

400 500 600 700 800 900 1000

Extin

ctio

n cr

oss s

ectio

n x

1015

(m2 )

Wavelength (nm)

References and Notes 1. R. L. Cropley et al., Efficient epoxidation of a terminal alkene containing allylic hydrogen

atoms: Trans-methylstyrene on Cu111. J. Am. Chem. Soc. 127, 6069 (2005). doi:10.1021/ja042758e Medline

2. A. K. Santra, J. J. Cowell, R. M. Lambert, Catal. Lett. 67, 87 (2000). doi:10.1023/A:1019053118841

3. J. J. Cowell, A. K. Santra, R. M. Lambert, Ultraselective epoxidation of butadiene on Cu111 and the effects of Cs promotion. J. Am. Chem. Soc. 122, 2381 (2000). doi:10.1021/ja994125j

4. R. M. Lambert, F. J. Williams, R. L. Cropley, A. Palermo, Heterogeneous alkene epoxidation: Past, present and future. J. Mol. Catal. Chem. 228, 27 (2005). doi:10.1016/j.molcata.2004.09.077

5. D. Torres, N. Lopez, F. Illas, R. M. Lambert, Low-basicity oxygen atoms: A key in the search for propylene epoxidation catalysts. Angew. Chem. Int. Ed. 46, 2055 (2007). doi:10.1002/anie.200603803

6. J. R. Monnier, G. W. Hartley, Comparison of Cu and Ag catalysts for epoxidation of higher olefins. J. Catal. 203, 253 (2001). doi:10.1006/jcat.2001.3336

7. S. Linic, P. Christopher, D. B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy. Nat. Mater. 10, 911 (2011). doi:10.1038/nmat3151 Medline

8. P. Christopher, H. Xin, S. Linic, Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures. Nat. Chem. 3, 467 (2011). Medline

9. P. Christopher, H. Xin, A. Marimuthu, S. Linic, Singular characteristics and unique chemical bond activation mechanisms of photocatalytic reactions on plasmonic nanostructures. Nat. Mater. 11, 1044 (2012). Medline

10. K. P. Rice, E. J. Walker, Jr., M. P. Stoykovich, A. E. Saunders, Solvent-dependent surface plasmon response and oxidation of copper nanocrystals. J. Phys. Chem. C 115, 1793 (2011). doi:10.1021/jp110483z

11. G. H. Chan, J. Zhao, E. M. Hicks, G. C. Schatz, R. P. Van Duyne, Plasmonic properties of copper nanoparticles fabricated by nanosphere lithography. Nano Lett. 7, 1947 (2007). doi:10.1021/nl070648a

12. E. M. Larsson, C. Langhammer, I. Zorić, B. Kasemo, Nanoplasmonic probes of catalytic reactions. Science 326, 1091 (2009). doi:10.1126/science.1176593 Medline

13. D. Seo, G. Park, H. Song, Plasmonic monitoring of catalytic hydrogen generation by a single nanoparticle probe. J. Am. Chem. Soc. 134, 1221 (2012). doi:10.1021/ja2093663 Medline

14. C. Novo, A. M. Funston, P. Mulvaney, Direct observation of chemical reactions on single gold nanocrystals using surface plasmon spectroscopy. Nat. Nanotechnol. 3, 598 (2008). doi:10.1038/nnano.2008.246 Medline

15. M. Hara et al., Cu2O as a photocatalyst for overall water splitting under visible light irradiation. Chem. Commun. Camb. 3, 357 (1998). doi:10.1039/a707440i

16. S. K. Cushing et al., Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor. J. Am. Chem. Soc. 134, 15033 (2012). doi:10.1021/ja305603t Medline

17. P. Christopher, D. B. Ingram, S. Linic, Enhancing photochemical activity of semiconductor nanoparticles with optically active Ag nanostructures: Photochemistry mediated by Ag surface plasmons. J. Phys. Chem. C 114, 9173 (2010). doi:10.1021/jp101633u

18. D. B. Ingram, S. Linic, Water splitting on composite plasmonic-metal/semiconductor photoelectrodes: Evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface. J. Am. Chem. Soc. 133, 5202 (2011). doi:10.1021/ja200086g Medline

19. D. B. Ingram, P. Christopher, J. L. Bauer, S. Linic, Predictive model for the design of plasmonic metal/semiconductor composite photocatalysts. ACS Catal. 1, 1441 (2011). doi:10.1021/cs200320h

20. K. Awazu et al., A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. J. Am. Chem. Soc. 130, 1676 (2008). doi:10.1021/ja076503n Medline

21. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters (Springer, New York, ed. 1, 1995).

22. B. N. J. Persson, Polarizability of small spherical metal particles: Influence of the matrix environment. Surf. Sci. 281, 153 (1993). doi:10.1016/0039-6028(93)90865-H

23. U. Kreibig, Interface-induced dephasing of Mie plasmon polaritons. Appl. Phys. B 93, 79 (2008). doi:10.1007/s00340-008-3213-1

24. H. Hövel, S. Fritz, A. Hilger, U. Kreibig, M. Vollmer, Width of cluster plasmon resonances: Bulk dielectric functions and chemical interface damping. Phys. Rev. B Condens. Matter 48, 18178 (1993). doi:10.1103/PhysRevB.48.18178 Medline

25. K. Watanabe, D. Menzel, N. Nilius, H.-J. Freund, Photochemistry on metal nanoparticles. Chem. Rev. 106, 4301 (2006). doi:10.1021/cr050167g Medline

26. L. Brus, Noble metal nanocrystals: Plasmon electron transfer photochemistry and single-molecule Raman spectroscopy. Acc. Chem. Res. 41, 1742 (2008). doi:10.1021/ar800121r Medline

27. A. M. Michaels, L. Jiang, Brus. J. Phys. Chem. B 104, 11965 (2000). doi:10.1021/jp0025476

28. A. Furube, L. Du, K. Hara, R. Katoh, M. Tachiya, Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles. J. Am. Chem. Soc. 129, 14852 (2007). doi:10.1021/ja076134v Medline

29. Y. Tian, T. Tatsuma, Plasmon-induced photoelectrochemistry at metal nanoparticles supported on nanoporous TiO2. Chem. Commun. (Camb.) (16): 1810 (2004). doi:10.1039/b405061d Medline

30. O. P. H. Vaughan, G. Kyriakou, N. Macleod, M. Tikhov, R. M. Lambert, Copper as a selective catalyst for the epoxidation of propene. J. Catal. 236, 401 (2005). doi:10.1016/j.jcat.2005.10.019

31. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, New York, 1985).