A composite CdS/ZnS core/shell catalyst for photocatalytic hydrogen generation and organic wastewater treatment under visible light

Transcription

1 2 + A composite CdS/ZnS core/shell catalyst for photocatalytic hydrogen generation and organic wastewater treatment under visible light e - h + O 2 2 O X. Wang and X.Y. Li Department of Civil Engineering The University of ong Kong 11 November 2010

2 World energy consumption 500 Global energy consumptuion ( 1016 kj) Petroleum Dry Natural Gas Coal ydroelectric Nuclear Petroleum Dry Natural Gas Coal ydroelectric Nuclear Net Geothermal, Solar, Wind, and Wood % 6% 1% % 26% % 2

3 Energy related issues No No x 6% CO 2 C 4 CFC s 56% 18% 13% Limited Energy Resource Energy consumption increases steadily Non-renewable energy - Fossil fuel petroleum, coal, natural gas Environmental Problems Air Pollution Greenhouse Gas Emission CO 2, NO x Acid Rain NO x, SO x 3

4 Solar energy Clean and renewable Solar Solar Energy Energy VS. VS. Electricity 10 Electricity Consumption Consumption Solar energy ( kj) Solar energy ( kj) Solar Energy Electricity Consumption Solar Energy Electricity Consumption Electricity Consumption ( kj) Electricity Consumption ( kj) 0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ong Kong 0 0 4

5 Photocatalytic hydrogen generation from water Solar energy ydrogen e hν>e g CB conduction band e - h + Band gap E g valence band VB 2 2 O 1.23eV 2 O O 2 E 2 / 2 O E O 2 / 2 O h + 2 O O 2 Photocatalytic hydrogen generation by semiconductor catalysts Challenge: visible light-driven catalysts 5

6 Catalysts for photocatalytic hydrogen production Most efficient photocatalysts:cds Composite catalyst Porous Pt/CdS 2300 µmol/hr (Bao et al., 2008) Mo 2 S/CdS 490 µmol/hr (Zong et al.,2008) Pt/PdS/CdS 4200 µmol/hr (Yan et al., 2009) Solid solution Cd x Zn 1-x S 180 µmol/hr (Zhang et al., 2007) Cd x In 2x Zn 2 S µmol/hr (Tsuji et al., 2006) Cd x Cu y Zn 1 x y S 550 µmol/hr (Liu et al., 2008) 6

7 Simultaneous organic degradation Organic functions as electron donors Visible light UV 7

8 Objectives Synthesis of CdS-ZnS nanoparticle catalysts with a core-shell structure for photocatalytic hydrogen generation from water under visible light. Utilization of the oxidation capability of the photocatalytic process for organic pollutant degradation together with photohydrogen evolution. Core-shell structure CO 2 - CdS core: small band gap, photo-sensitive but less stable. - ZnS shell, larger band gap, less photo-sensitive but more stable. organics 8

9 Methods 9

10 Preparation of core-shell nanocatalysts Microemulsion method: (CdS) x /(ZnS) 1-x 50x ml 0.1M Cd(NO 3 ) 2 microemulsion Core-shell (CdS) x /(ZnS) 1-x 50ml 0.1M Na 2 S microemulsion S 2- S 2- S2- CdS 2- S 2- S 2- S Stirred for 15min and the bright yellow solution formed 50(1-x) ml 0.1M Zn(NO 3 ) 2 microemulsion Aging 12h at room temperature Centrifuged and washed with water and absolute ethanol Dry at 50 C in vacuum condition for 6h Different annealing temperatures in N 2 for 2 hr: 295 K, 723 K, 873 K Photo-deposition of noble metals: Ru, Pt, Pd doping 10

11 Characterization of the catalysts ultrasonicated for 30 min LS Series Laser Diffraction Particle Size Analyzer Leo 1530 FEG Particle size distribution SEM TEM UV-Vis diffuse reflectance spectra Philips Tecnai G2 20 S-TWIN UV/Vis Spectrophotometer (Lambda 25, Perkin Elmer, Wellesley, MA, USA). 11

12 ydrogen production tests Methods Light source reactor Gas collector Light source: Irradiation area: 33 cm 2 Max light intensity: 70.4 mw/cm 2 (Visible light) 86.4 mw/cm 2 (UV + Visible light) Model organic: Formic acid Methanol Ethanol (vs. S 2- /SO 2-3 ) 12

13 Photo-reactivity evaluation Methods Photocatalytic hydrogen production reactivity m 2 2 production rate = T Energy conversion efficiency η = 2 production I A rate c m 2 2 specific production rate= T A m 2 - moles of hydrogen evolution; ΔT - duration of reaction; A - light irradiation area. Δ c - combustion value of hydrogen (286 kj/mol); I - light intensity. 13

14 Results and Discussion 14

15 Synthesis (CdS) x /(ZnS) 1-x Catalysts x: 1 0 (No heating treatment) Results and Discussion 15

16 Characterization: Size (CdS) x /(ZnS) 1-x x Mean Size/nm Median Size/nm Number (%) The size of the 0.04 catalysts 0.09 is 0.14 independent 0.19 of 0.24 the catalyst composition. It is controlled by the droplet size Paticle during size (µm) the microemulsion process. 16

17 SEM Characterization: Morphology SEM The particle size range from 100 nm to 400 nm The irregular shape and surface indicates the aggregation of primary particles Results and Discussion 17

18 TEM Characterization: Morphology The photocatalysts are nano-particles with a size of around 10 nm. The nano-sized catalysts aggregated into larger particle clusters. 18

19 Characterization: photophysical properties (CdS) x /(ZnS) 1-x K.M. Absorbance E ( ev ) Band Gap Eg (ev) g = 1240 λ g UV Visible ( nm) x in (CdS)x/(ZnS)1-x Wavelength (nm) When the ratio factor x varies from 0.9 to 0.1, the catalysts can be excited by visible light, with apparent band gaps of ev 19

20 Photocatalytic hydrogen production ydrogen production: S 2- /SO 3 2- Specific hydrogen production rate µmol/cm2-h Visible light x in (CdS) x /(ZnS) 1-x UV and visible light The ZnS shell improved the response of the catalysts to visible light and increased the photo-hydrogen productivity. 20

21 Effect of the annealing temperature ydrogen production: S 2- /SO ydrogen production rate (µmol/cm 2 -h) Visible light UV and visible light Max. 2 production rate :17.4 µmol/cm 2 - h Annealing temperature (K) 21

22 Effect of the annealing temperature XRD pattern Intensity (a.u.) C C C C C C C C: cubic : hexagonal 873 K 723 K C C 295 K Good crystallization and formation of the hexagonal phase at a high temperature are likely contributable to the increased hydrogen production rate θ (degree) 22

23 Effect of the annealing temperature 295 K 723 K 873 K owever, a too high temperature would increase the size of catalyst particles m 2 /g m 2 /g m 2 /g from 10 nm to100 nm, resulting in a lower rate of photocatalytic 2 production. 23

24 Noble metal doping Noble metal doping for visible light reactivity 70 ydrogen Production Rate (µmol/cm 2 -h) Ru Pt Pd Catalyst 0.5%Ru/CdS/ZnS 0 exhibitd the highest 2 evolution activity at 57.5 µmol/cm 2 0 -h, with an energy 0.5 conversion 1 efficiency 1.5 of 6.5%. Loading (wt%) 24

25 ydrogen production together with photocatalytic organic degradation ydrogen produciton (µmul) Formic acid Methanol Ethanol Water Formic acid Ru Methanol Ru Ethanol Ru Water Ru ydrogen production rate: Formic Time (h) acid>methanol>ethanol as electron donors. 25

26 ydrogen production with simultaneous organic degradation Theoretic COD removal rate (mg/cm 2 -hr) CdS/ZnS Ru/CdS/ZnS Degradation of the model organics can be achieved during the 0 photocatalytic process with a maximum COD removal rate of formic acid methanol ethanol 0.4 mg/cm 2 -h. 26

27 ydrogen production with simultaneous organic degradation Carbon dioxide evolution (µmol) catalysts CdS/ZnS Formic acid Methanol Ethanol Water Ru/CdS/ ZnS ydrogen evolution (µmol) Formic acid Methanol Ethanol Water CdS / COO ZnS CdS/ZnS hv h Ru/CdS/ZnS + + e + hv + h CO hv + e

28 Conclusions (CdS) 0.8 /(ZnS) 0.2 with an aging time of 6 h followed by annealing at 723K exhibits the highest hydrogen productivity at a rate of 17.4 µmol/cm 2 -h under visible light. Meanwhile, degradation of model organic pollutants, e.g. formic acid, ethanol and methanol, can be achieved during the photocatalytic process with a maximum COD removal rate of 0.4 mg/cm 2 -h. R&D in environmental nanotechnology are expected to lead to technological breakthroughs for renewable energy and sustainable water pollution control. 28

29 Acknowledgement: - SRT and URC funding from The University of ong Kong - Special Equipment Grant SEG_KU10 from the University Grants Council (UGC) of the ong Kong SAR Government 29

30 TANK YOU! 30