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Electronic Supplementary Material (ESI) for Chemical Communications This journal is © The Royal Society of Chemistry 2013

Electronic Supporting Information (ESI) Selective Photocatalytic Conversion of Glycerol in Aquesou Solution to Hydroxyacetaldehyde on Facet Tuned TiO2–based Catalysts Ruifeng Chong, a,b Jun Li, a Xin Zhou,a Yi Ma, a Jingxiu Yang, a Lei Huang, a Hongxian Han, a Fuxiang Zhang a and Can Li a* a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian, 116023 (China) b

Graduate University of Chinese Academy of Sciences, Beijing, 100049, China.

E-mail: [email protected]

Homepage: http://www.canli.dicp.ac.cn

Fax: 86-411-84694447 

These authors contributed equally to this work.

1


Experimental section 1) Preparation of TiO2 with different facets A{001} was synthesized by modifying a method reported in the literature.S1 In a typical synthesis, titanium isopropoxide (TTIP, 97%, Alfa Aesar, 10 mL) and hydrofluoric acid solution (1.5 mL, 40 wt%) were mixed in a Telfon-lined 50 mL autoclave at room temperature, and then kept at 473 K for 24 h. A{101} and R{110} were prepared by a hydrothermal procedures as reported in a previous work.S2 The desired volumes (1 mL and 6 mL, respectively) of HCl (36.5 %) and 15 mL of titanium isopropoxide were mixed in a Telfon-lined 100 mL autoclave at room temperature and then kept at 453 K for 36 h. After the autoclave was cooled to room temperature, the resulting white precipitates were separated and washed with 5 mol L-1 NaOH solution for several times, followed by washing with distilled water until the filtrate being neutral. The final products were dried in an oven at 353 K overnight. 2) Photocatalytic experiments Experiments of photocatalytic reaction at 293 K or 353 K were carried out in a Pyrex reaction cell, connected to a closed gas circulation and a vacuum system. A 300 W top-irradiated Xenon lamp was used as a light source. Prior to illumination, the system was deaerated by evacuation in order to remove the air in the system. Typically, 0.1 g of catalyst was suspended in a 100 mL glycerol solution. 0.1wt% Rh, (or Pt, Pd, Au, Ru) or 1.0 wt% Cu (or Ni) was loaded as a co-catalyst on TiO2 at the beginning of the reaction by an in-situ photodeposition method using the precursor of RhCl3 (or H2PtCl6·6H2O, PdCl2, H2AuCl6·6H2O, RuCl3, CuCl2, Ni(NO3)2). 2


3) Hydrogenation process In this experiment, 5 mL of the reaction mixture filtrated after photocatalytic reaction and commercial 5 % wt Ru/C (Alfa, 50% in water, 10 mg) were mixed in a test tube, which was put in a 100 mL of stainless-steel autoclave. The autoclave was purged with H2 for 3-4 times. The final H2 pressure was set at 5 bar. The hydrogenation reactions were prformed for 2 h at room temperature under magnetically stirring (r = 900 min-1). 4) Computational Method Density functional theory calculations employing B3LYP/6-31G(d) methodS3 provided in Gaussian 09S4 are conducted to investigate the interaction between water and TiO2 surfaces. The unit cells of anatase and rutile TiO2 are formed by employing the experimental parameters of bulk titanium dioxide.S5 In order to simulate the effect of the photocatalytic hole finite cationic cluster models of Ti9O33H30 and Ti11O40H36 representing anatase {001} and rutile {110}, titanium dioxide surfaces are obtained from the enlarged unit cell by saturating the peripheral oxygen atoms with hydrogen atoms. This cluster approach is a well known and successful approach applied in quantum chemical calculations.S6 Computations have been carried out for partially relaxed cluster representation. All of the neighbors of the surface Ti atoms that bind adsorbed water molecule are optimized and the rest of atoms are fixed. Vibrational frequencies were calculated to characterize the stationary points or transition states located on the potential energy surface and to obtain the zero-point energy corrections as well as the thermal and free energy corrections at 298 K. 3


5) Analytical techniques X-ray powder diffraction (XRD) was carried out with a Rigaku D/Max-2500/PC powder diffractometer using CuKa radiation source at a scaning rate of 5°/ min in the 2θ range of 20-80°at a step size of 0.02 s under an operating voltage of 40 kV and an operating current of 200 mA. UV Raman spectra were collected on a home-made Raman spectrograph system, the spectrograph is triple-stage dispersed-subtractive spectrograph, in which the first two stages are used to cut off the Rayleigh line and the third one is used to collect Raman spectra using CCD detector. The scattered lights were collected by the ellipse collecting mirror in a back-scattering geometry and focused into the entrance of the Raman spectrograph. The Raman spectra were recorded with a spectral resolution of 2 cm−1 with the laser excitation at 325 nm from He-Cd Laser. Transmission electron microscopy (TEM) images were taken on a Tecnai G2 Spirit (FEI company) using an accelerating voltage of 120 kV. High-resolution transmission electron microscopy (HRTEM) images were performed on Tecnai G2 F30 S-Twin (FEI company) with an accelerating voltage of 300 kV. The Brunauer-Emmett-Teller (BET) surface area was measured based on nitrogen adsorption at 77 K using a Micromeritics ASAP 2000 adsorption analyzer. X-ray photoelectron spectroscopy (XPS) were recorded on a KROTOS AMICAS spectrometer (Shimadzu, Japan) with a magnesium Ka (hn=1253.6 eV) radiation source, using C 1s (284.6 eV) as reference. Electron

paramagnetic

resonance

(EPR)

of

radicals

trapped

by

5,5-dimethyl-1-pyrroline N-oxide (DMPO) were recorded on a Brucker EPR A200 4


spectrometer. The samples containing 1 g·L-1 TiO2 (A{001}, A{101}, and R{110}, respectively) and 0.05 mol L-1 DMPO were vacuumized followed by ventilation with argon for 3 times. After that, the samples were introduced into a home-made quartz cup inside the microwave cavity and illuminated with a 300 W Xe lamp (CERAMAX LX-300). The settings for the EPR spectrometer were as follows: center field, 3350 G; sweep width, 80 G; microwave frequency, 9.41 GHz; modulation frequency, 100 kHz; power, 2.00 mW. The qualitative analysis of product (HAA) was performed on GC-MS system (Agilent 6890 N GC/5973 MS detector) using helium as carrier gas under split model (60:1). Typically, the inject amount is 1 μL) with an autosampler. The temperature of the column was initially kept at 373 K for 2 min, and the column temperature was then increased at a rate of 20 K/ min to 523 K. The qualitative analysis of HAA was further detected by 1H NMR. 1H NMR spectrum was recorded on a Bruker DRX 400 MHz type (1H, 400 MHz) with an internal reference tetramethylsilane. Typically, the mixture was evaporated to remove water after reaction. 2,4-dinitrophenylhydrazine (0.5 equiv.) in ethanol (5 mL ) was added and stirred for 12 h. The solution was extracted with EtOAC (3×5 mL). After removing organic solvent, the yellow solid was washed with water and dried in vacuum. 1H NMR (400MHz, CDCl3) δ4.51 (s 2H), 7.67 (s, 1H ),7.94 (d, J=8 Hz, 1H), 8.35 (d, J = 8 Hz, 1H), 9.14 (s, 1H). The gaseous products from glycerol-water mixture, such as H2 and CO2, were analyzed by an online gas chromatography. The amount of hydrogen produced was 5


measured by a thermal conductivity detector and CO2 was detected by a flame ionization detector after it was converted into CH4 in a methanator. The liquid products quantitative analysis was carried out using HPLC (Agilent 1200) with refractive index (RI) detector and ultraviolet (UV) detector for glycerol, GA, HAA and FA. Reactant and products were separated through an ion exclusion column (Alltech OA-1000) heated at 303 K. The eluent was a solution of H2SO4 (5 mmol L-1). Products were identified by comparison with standard samples. Glycerol (>99.5%), GA, HAA and FA were obtained from Sigma–Aldrich. In this work, the conversion of glycerol and the selectivity of HAA are defined as follows: Conversion =

Selectivity =

Cr0  Cr 100% Cr0

Cp Cr0  Cr

100%

cr0 : the initial concentration of the reactant; cr : the concentration of the reactant during the reaction; cp: the concentration of the product during the reaction.

6


Figures and Tables

Fig. S1 XRD patterns of the samples of A{001}, A{101} and R{110}, and the structure models of a) anatase {001}, b) anatse {101} and c) rutile {110} facet. On the anatase {001} facet, all titanium atoms are five-coordinate (Ti5c) with a Ti-O-Ti angle of 146° (Fig. S1a); the sawtooth {101} facet has half of five-coordinate (Ti5c) with a Ti-O-Ti angle of 102° (Fig. S1b)S7 while rutile {110} surface is flat with O2c bound to Ti6c and the rows of Ti5c running parallel to the rows of bridging oxygens (Fig. S1c) S8. 7


O1s

Intensity (a.u.)

Ti 2p

A{001} A{101}

R{110} 1000

800

600 400 200 Binding Energy (eV)

0

Fig. S2 XPS patterns of samples A{001}, A{101} and R{110}. The binding energy of F1s electron is 684 eV and Cl 2p electron is 198.4 eV. There is no such peak for each sample. It indicates that A{001} is fluoride-free, and both A{101} and R{110} are chloride-free .

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Table S1: The blank experiments of the reaction of glycerol.

Entries

Catalyst

1

h

Temp. (K)

Time (h)

Conv. (%)

√

353

3

0

2

√

353

3

0

3

√

473

3

0

Reaction conditions: 0.02 mol L-1 aqueous glycerol solution, 10 mL; catalyst: 0.05g 0.1wt% Rh/R{110}.

Table S2: The yields of the products in photocatalytic conversion of glycerol.

Rh/TiO2

Products (umol)

Consumed glycerol (umol)

CO2

GA

HAA

FA

A{001}

130

2.7

42

280

273

A{101}

178

6.8

173

63

356

R{110}

9

4

393

206

410

Reaction conditions: catalyst, 0.1 g; Co-catalyst, 0.1 wt% Rh; 0.02 mol L-1 aqueous glycerol solution, 100 mL; Light source, Xe-lamp (300 W); Reaction temperature, 353 K. Reaction time, for A{001} and A {101},4h; R{110}, 2h.

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Fig. S3 The mass Spectra of HAA

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Fig. S4 The 1H NMR of HAA after derivation

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Table S3: Photocatalytic conversion of glycerol in aqueous solution with R{110} loaded with different cocatalysts (M).

H2 (μmol)

HAA (μmol)

nH2/nHAA

none

138

67

2.06

Pt

2453

1218

Rh

2242

Pd

M

Conv. (%)

Sel. (%) nCO2/nH2 HAA

GA

﹤1

97.7

2.3

0.018

2.01

11.6

95.7

3.0

0.070

1054

2.13

10.3

93.2

2.6

0.049

1417

655

2.16

7.1

92.0

6.5

0.069

Ru

523

244

2.14

2.4

90.8

3.2

0.024

Au

960

466

2.06

4.7

90.2

1.5

0.046

Cu

1629

802

2.03

8.0

91.2

4.6

0.065

Ni

1441

740

1.95

7.2

93.5

2.8

0.043

Reaction conditions: TiO2, 0.1 g; Co-catalyst, 0.1 wt% Pt, Rh, Pd, Au, Ru, 1 wt% Cu, Ni; 0.11 mol L-1 aqueous glycerol solution, 100 mL; Light source, Xe-lamp (300 W); Reaction temp., 293 K; Reaction time, 8 h.

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Table S4: The conversion of HAA and the main products

Products (mM) Conv. (%)

6.2

FA

HCHO

CO2

4.5

5.1

1.4

Reaction conditions: Rutile, 0.1 g; Co-catalyst, 0.1 wt% Rh; 0.11 mol L-1 aqueous HAA solution, 100 mL; Light source, Xe-lamp (300 W); Reaction temp., 293 K; Reaction time, 8 h.

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Figure S7. Typical EPR spectra for photocatalytic oxidation of H2O on TiO2 (blue lines) and the simulated EPR spectra of the DMPO-OH and DMPOX radical were shown as the red lines.

Fig. S5 Typical EPR spectra for photocatalytic oxidation of H2O on TiO2 (blue lines) and the simulated EPR spectra of the DMPO-OH and DMPOX radical (red lines).

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a) Energtics of H2O Dissociation on Anatase {001} H2O +

0 eV

-0.38 eV

-1.69 eV 1st O–H Dissociation

-1.29 eV 2nd O–H Dissociation

b) Energtics of H2O Dissociation on Rutile {110} H2O +

0.0 eV - 0.35 eV

-0.29 eV

-0.26 eV - 0.57 eV 1st O–H Dissociation

- 0.83 eV 2nd O–H Dissociation

Fig. S6 Potential energy surface for the two-step dissociation of H2O on TiO2. a) anatase {001} facets. b) rutile {110} facets. Relative energies calculated at the B3LYP/6-31G(d) level provided in Gaussian 09. As shown in Fig. S6a, the first O−H bond can undergo a barrierless dissociation on Ti5c site of anatase {001}. The second O−H dissociation step to produce O adatom is calculated to be endothermic by 0.4 eV with a quite high barrier, 1.31 eV. The results suggest that the lowest energy state of the dissociated H2O on anatase {001} facets has hydroxyl bound to a Ti5c site. On the other hand, the calculated adsorption energy for H2O on rutile {110} facets is exothermic by 0.35 eV (Fig. S6b). The first O−H dissociation energy is 0.22 eV with a very low barrier, 0.06 eV. The second O−H dissociation step is computed to be exothermic by 0.26 eV with a barrier of 0.31 eV, which results in the lowest energy state of O adatom (-0.83 eV) on rutile {110} facets.

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a)

80

30 60

20

40

10

HAA sel. (%)

Glycerol Conv. (%)

40

20

0

0

0

1

2

3

4

5

t/h

Fig. S7 Two-step process for glycerol-water mixture conversion to EG. a)The conversion ratio of glycerol and selectivity of HAA versus the reaction time; b) Catalytic hydrogenation of the intermediate HAA to EG on Ru/C. Reaction conditions of a): catalyst, R{110} 0.1 g; Co-catalyst, 0.1 wt% Rh; 0.02 mol L-1 aqueous glycerol solution, 100 mL; Light source, 300 W Xe-lamp; Reaction temperature, 353 K.

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