Efficient Visible Light Photocatalytic CO 2 Reforming of CH 4

Transcription

1 SUPPORTING INFORMATION Efficient Visible Light Photocatalytic CO 2 Reforming of CH 4 Bing Han, Wei Wei, Liang Chang, Peifu Cheng, and Yun Hang Hu* Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI , USA * yunhangh@mtu.edu CONTENT: 1. Experiment Section 2. Band potential measurement Fig. S1 (EPR spectra of Pt/ blacktio 2, Pt/non-black TiO 2, and white TiO 2 ) Fig. S2 (FT-NIR spectra and the first-order derivative FT-NIR spectra of Pt/blackTiO 2, Pt/ non-blacktio 2, and white TiO 2 ) Fig. S3 (Mott-Schottky plots of Pt/ blacktio 2, Pt/non-black TiO 2, and white TiO 2 ) Table S1 (Band potentials) 3. Light-diffuse-reflection surface of SiO 2 substrate 4. SEM image and UV-visible light absorption Fig. S4 (FESEM image of Pt/ blacktio 2 coated on SiO 2 substrate) Fig. S5 (UV-Visible spectra of Pt/ black TiO 2, Pt/non-black TiO 2, and white TiO 2 on SiO 2 substrate) 5. Photocatalytic reactor Fig. S6 (Photocatalytic reactor) 6. Effect of CH 4 /CO 2 ratio on photocatalysis 1

2 Fig. S7 (Photo product yields vs. CH 4 /CO 2 ratio) 7. Effect of light intensity on photocatalytic CO 2 reforming of methane Fig. S8 (Photo product yields vs. light intensity) 8. Control experiments comparing black Pt/TiO 2 and TiO 2 for photocatalytic CO 2 reforming of methane Fig. S9 (Comparison of Pt/black TiO 2, Pt/non-black TiO 2, and white TiO 2 for photocatalytic CO 2 reforming of methane) 9. Catalyst stability Fig. S10 (Photo product yields vs. reaction time) Fig. S11 (TEM images and EPR spectra) Fig. S12 and Table S2 (XRD patterns of catalysts) 10. References 2

3 1. EXPERIMENTAL SECTION Preparation and characterization of a SiO 2 substrate: Silicon dioxide and quartz wool were mixed with water at weight ratio of 30:30:1 and then heated to 1100 C for 2h to form a piece of white solid. The solid piece was shaped as a rectangle disk substrate (1.4 cm 2.9 cm). Its excellent light scattering properties were demonstrated using Mahr Federal Pocket Surf III profilometer and Shimadzu UV-Vis spectrophotometer (Figure S5 in the supporting information). Pt/BlackTiO 2 catalyst and its dispersion on light-diffuse-reflection-surface: 1wt% Pt/TiO 2 catalyst was synthesized by impregnating TiO 2 powder (Degussa P25 with surface area of 48 m 2 /g) in aqueous solution of H 2 PtCl 6 6H 2 O at 25 o C overnight, followed by calcination at 500 o C for 2 hours. The obtained Pt/TiO 2 powder, which was located in a ceramic tube reactor (diameter of 3 cm), was vacuumed for 6 hours. Then, hydrogen was introduced into the reactor, followed by increasing temperature to 200 o C in 20 min and remaining the temperature for 24 hours. Consequently, Pt/black TiO 2 catalyst was generated. To disperse the catalyst on the light-diffusereflection-surface, Pt/blackTiO 2 powder was mixed with water as a paste, followed by coating on the surface of the SiO 2 substrate and then drying at 25 o C overnight. The characterization of catalysts (including band structures) was provided in supporting information (Figures S1-S5, S11, and S12). Photocatalytic CO 2 reforming of methane: Pt/BlackTiO 2 catalyst (15 mg) on the light-diffusereflection-surface of a SiO 2 rectangle disk substrate (1.4 cm 2.9 cm) was located in a quartz reactor (diameter: 1.5 cm). CH 4 /CO 2 mixture gas (1:1 ratio and GHSV=40000 ml/gcat/h) flow was introduced into the reactor located in an electrical furnace (Figure S6 in supporting information). The photocatalytic reaction at a selected temperature was carried out under 3

4 illumination of 1 Sun (100 mw/cm 2, simulated by AM 1.5G Newport solar simulator). To eliminate the effect of light illumination on the temperature of catalyst, an in-situ thermocouple was employed for accurate temperature-control. For visible light photocatalytic reaction test, UV light filter (wavelength < 420 nm) was exploited to eliminate UV light from AM 1.5G sunlight. On-line gas chromatograph equipped with Porapak Q and 5A columns was used for gas product analysis and GC-Mass spectrometer for condensed liquid product analysis. The measurements showed that the products were H 2, CO, and H 2 O. Structure characterization: X-ray diffraction (XRD) patterns for catalysts were obatined using Scintag XDS-2000 powder diffract meter with Cu Kα (λ=1.5406å) radiation. The TEM and SEM images of the catalysts were obtained using JEOL JEM2010 transmission electron microscopy (TEM) and Hitachi S-4700 field emission scanning electron microscopy (FE-SEM). Electron paramagnetic resonance (EPR) spectra of the catalysts were recorded at 77k by a commercial continuous wave (CW) X-band (9-10 GHz) EPR spectrometer ESP-300E from Bruker. Apparent quantum efficiency: In photocatalytic CO 2 reforming of methane, the reduction reaction is CO 2 to CO, whereas CH 4 to CO (and/or H 2 O) is an oxidation reaction. The formation of H 2 from CH 4 doesn t include the electron transfer. Because the conversion of each CO 2 molecule to CO requires 2 electrons, the apparent quantum efficiency can be expressed as follows: ( number of photo converted CO2 molecules) 2 Apparent quantum efficiency= number of incident photons (1) 4

5 The measurements of incident photons were carried out using Newport Quantum Efficiency Test Instrument under the same illuminations (visible light or AM 1.5 global sunlight) as used for the photocatalytic reactions. 2. BAND POTENTIAL MEASUREMENT Electron paramagnetic resonance (EPR) measurements were carried out for pristine (white) TiO 2, Pt/non-blackTiO 2, and Pt/blackTiO 2. The spectra were recorded at 77 K. It is wellknown that a peak at g = 1.95~1.98 is characteristic of a paramagnetic Ti 3+ center S1. As shown in Fig. S1, the peak intensity of Ti 3+ is very small for white TiO 2 and unreduced Pt/TiO 2, indicating their negligible amount of Ti 3+. In contrast, the Pt/blackTiO 2 exhibits a large amount of Ti 3+ associated with a large peak of Ti 3+. Figure S1. EPR spectra of (a) pristine (white) TiO 2, (b) Pt/nonblackTiO 2 without hydrogenation, and (c) Pt/blackTiO 2 prepared by hydrogenation (Ref. 30). The Fourier transform near infrared (FT-NIR) spectrum of the Pt/blackTiO 2 (prepared by hydrogenation) was determined using a Bomem (MB 160) Fourier transform near infrared spectrophotometer in the wavenumber range cm -1 at room temperature. Polytetrafluoroethylene (PTFE) was used as the reference material. The obtained (FT-NIR) 5

6 spectrum was shown in Fig. S2A. For comparison, FT-NIR spectra were also recorded for white TiO 2 and Pt/non-blackTiO 2 (without hydrogenation). The spectra show that Pt/blackTiO 2 exhibited strong absorption in the wavenumber range of cm -1, whereas obvious absorption was not observed for white TiO 2 and Pt/non-blackTiO 2 (without hydrogenation). Furthermore, the first-order derivative FT-NIR spectra was obtained for Pt/blackTiO 2 from its spectrum (Fig. S2B). It was well-demonstrated that the first-order derivative FT-NIR spectrum can provide an accurate method to determine the energy gap, namely, the peak position of the first-order derivative spectrum is corresponding to an energy gap S2-S4. As shown in Fig. S2B, the peak position of the first-order derivative of FT-NIR signal for Pt/blackTiO 2 is at cm -1, corresponding to an energy gap of 1.3 ev. In contrast, white TiO 2 and Pt/non-blackTiO 2 don t absorb light in the wavenumber range. Furthermore, EPR spectra show that Ti 3+ is present in the Pt/ blacktio 2, but not in white TiO 2 and Pt/non-blackTiO 2 (Fig. S1). This indicates that an energy gap of 1.3 ev of the Pt/blackTiO 2 would be due to the Ti 3+ donor level. In other words, the energy gap between the conduction band (CB) and the generated donor level (Ti 3+ ) for Pt/blackTiO 2 is 1.3 ev. (A) (B) 6

7 Figure S2. (A) FT-NIR spectra of (a) Pt/black TiO 2 prepared by hydrogenation, (b) Pt/non-black TiO 2 without hydrogenation, and (c) white TiO 2 ; and (B) the first-order derivative FT-NIR spectra of Pt/black TiO 2 prepared by hydrogenation. The band potentials of pristine (white) TiO 2 (P25), Pt/non-black TiO 2, and the Pt/black TiO 2 catalyst were measured as follows: (a) Flat-band potentials (E fb ) were determined by Mott- Schottky method with 0.1 M LiClO 4 as electrolyte, Pt as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode for both 1 cm 2 pristine (white) TiO 2 coated on a fluorine-doped tin oxide (FTO) glass plate and 1 cm 2 Pt/blackTiO 2 coated on a FTO glass plate. Argon (>99.99%) was bubbled through the electrolyte to remove oxygen before the test. The capacitance was measured by electrochemical impedance spectroscopy (Fig. S3). Conduction band edge potentials (E cb ) were calculated from flat-band potentials with a method described in ref. S5. (b) The energy gap between conduction band (CB) and the generated donor level (Ti 3+ ) for Pt/blackTiO 2 was determined using a Bomem Fourier transform near infrared (FTNIR) spectrophotometer (Fig. S2), and the energy gaps (E g ) between conduction band (CB) and valence band (VB) for white TiO 2 and Pt/black TiO 2 were measured using a Shimadzu UV-Vis spectrophotometer (Fig. S5). (c) The valence band edge potential (E vb ) was obtained by combining the conduction band edge potential with the VB-CB energy gap. The obtained results are listed in Table S1. 7

8 (A) (B) (C) Figure S3. Mott-Schottky plots for (A) white TiO 2, (B) Pt/non-black TiO 2 without reduction, and (C)Pt/blackTiO 2 prepared by hydrogenation: The flat-band potentials were obtained from the intercepts of the extrapolated lines. Furthermore, conduction band edge potentials (E cb ) were calculated from flat-band potentials with a method described in ref. S5. Table S1. Band potentials of pristine TiO 2, Pt/non-black TiO 2 and Pt/black TiO 2. Potential (V vs SHE) Pristine TiO 2 Non-black Pt/TiO 2 Black Pt/TiO 2 Flat band potential (E fb ) Conduction band edge potential (E cb ) Energy gap (E g ) Valence band edge potential (E vb ) LIGHT-DIFFUSE-REFLECTION SURAFCE OF SiO 2 SUBSTRATE The surface roughness and the light scattering coefficient of our synthesized SiO 2 substrate are µm and 385 cm 2 /g, respectively. Furthermore, incident light was reflected at various angles instead of just a specular reflection angle, which was shown in our previous work 30. Those demonstrate that the synthesized SiO 2 substrate possesses an excellent light-diffusereflection-surface. 4. SEM IMAGE AND UV-VISIBLE LIGHT ABSORPTION The dispersion of the Pt/black TiO 2 catalyst on the light-diffuse-reflection-surface of the SiO 2 substrate was evaluated by a Hitachi-4700 field emission scanning electron microscope (FESEM) (Fig. S4). The light absorbance was determined using a Shimadzu UV-Visible spectrophotometer (Fig. S5). 8

9 Figure S4. FESEM image of Pt/black TiO 2 coated on the SiO 2 substrate. Figure S5. UV-Visible spectra of Pt/black TiO 2, Pt/non-black TiO 2, and white TiO 2 on the SiO 2 substrate. 9

10 5. PHOTOCATALYTIC REACTOR Figure S6. Test unit schematic for photocatalytic CO 2 reforming of methane. (1) Thermocouple, (2)Pt/Black TiO 2 on the SiO 2 substrate, (3) Quartz wool, (4) Quartz tube reactor, and (5) Electrical tube furnace. 10

11 6. EFFECT OF CH 4 /CO 2 RATIO ON PHOTOCATALYSIS Figure S7. Photo product yields of CO 2 reforming of methane over Pt/ black TiO 2 catalyst under AM 1.5 global sunlight illumination at 600 o C. Pure CH 4 (corresponding to 10/0 of CH 4 /CO 2 ratio) can produce small amount of H 2, whereas no reaction can take place for pure CO 2 (corresponding to 0/10 of CH 4 /CO 2 ratio). The optimum CH 4 /CO 2 ratio is 1/1, which can generate the highest photo product yield. 11

12 7. EFFECT OF LIGHT INTENSITY ON PHOTOCATALYTIC CO 2 REFORMING OF METHANE Figure S8. Photo product yields of CO 2 reforming of methane over Pt/black TiO 2 catalyst on the light-diffuse-reflection-surface of the SiO 2 substrate under visible light illumination at 550 o C. 12

13 8. CONTROL EXPERIMENTS COMPARING Pt/BLACKTiO 2, Pt/NON-BLACKTiO 2 AND TiO 2 FOR PHOTOCATALYTIC CO 2 REFORMING OF METHANE Figure S9. Comparison of Pt/black TiO 2 catalyst, Pt/non-black TiO 2 catalyst, and pristine (white) TiO 2 for CO 2 reforming of methane under AM 1.5 global sunlight illumination at 700 o C. 9. CATALYST STABILITY 13

14 The variation of H 2 and CO yields with reaction time was evaluated for CO 2 reforming of methane under visible light illumination. As shown in Fig. S10, one can see that the activity decreased with reaction time, particularly in initial 5 hours. TEM (transmission electron microscopy) images showed that Pt particle sizes were the same before and after the reaction. Furthermore, no carbon deposition was observed by TEM. Those indicates that the deactivation was not due to Pt particle size change and carbon deposition. EPR measurements showed that the Ti 3+ content increased after the reaction (Fig. S11). Therefore, the deactivation was not due to the oxidation of Ti 3+ with CO 2. However, XRD patterns showed the transformation of black TiO 2, namely, the anatase phase decreased and the rutile increased with increasing reaction time (Fig. S12 and Table S2). It suggests that the deactivation was due to the phase transformation of TiO 2. Figure S10. Photo product yields (A. H 2 and B.CO) vs. reaction time of CO 2 reforming of methane over Pt/blackTiO 2 catalyst on the light-diffuse-reflection-surface of the SiO 2 substrate under visible light illumination at 600 o C. 14

15 Figure S11. TEM images of catalysts (A: Pt/black TiO2 catalyst, B: Pt/blackTiO2 catalyst used for photo CRM under 1.5G illumination for 20 hours at 600 oc, and C: Pt/blackTiO2 catalyst used photo CRM under visible light illumination for 20 hours at 600 oc) and EPR of the catalysts (D). 15

16 Figure S12. XRD patterns of Pt/blackTiO 2 catalysts used for photo CRM under visible light illumination at 600 o C. Table S2. Phase composition of Pt/blackTiO 2 (the content of Pt is 1 wt%) Catalyst Anatase (%) Rutile (%) Before reaction h* h h

17 *Reaction time for photo CRM under visible light illumination. 11. REFERENCES (S1) Xing, M.; Zhang, J.; Chen, F.; Tian, B. Chem. Commun. 2011, 47, (S2) Sotelo-Lerma, M.; Zingaro, R.; Castillo, S. J. Organomet. Chem. 2001, 623, (S3) Riveros, G.; Gomerz, H.; Henriquez, R.; Schrebler, R.; Marotti, R.; Dalchiele, E. Bol. Soc. Chil. Quimi. 2001, 47, (S4) Huo, Y.; Hu, Y. H. J. Phys. Chem. Solids. 2012, 73, (S5) Katz, M. J.; Vermeer, M. J. D.; Farha, O. K.; Pellin, M. J.; Hupp, J. T. Langmuir 2013, 29,