1. HYDROTALCITES AS PHOTO CATALYSTS FOR REDUCTION OF CARBON DIOXIDE D. Sushil Kumar - CA10M006, M. Tech in CATALYSIS TECHNOLOGY. NATIONAL CENTRE FOR CATALYSIS RESEARCH INDIAN INSTITUTE OF TECHNOLOGY MADRAS CHENNAI-60036 1

2. Hydrotalcites Magnesium-aluminum hydroxycarbonate, is a naturally occurring mineral of the chemical composition Mg 6 Al 2 (OH) 16 CO 3 ×4H 2 O This mineral exhibits a layered crystal structure similar to that of brucite, Mg(OH) 2, where each Mg 2+ cation is octahedrally surrounded by six hydroxyl groups and the adjacent octahedra share edges to form infinite sheets. The sheets are stacked one on top of the other and are held together by hydrogen bonding. In hydroxide sheets of hydrotalcite, an Mg 2+ /Al 3+ isomorphous substitution in the octahedral sites results in a net positive charge neutralized by interlayer an ionic species. S. Kannan ; Catalysis Surveys from Asia, Vol. 10, Nos. 3/4, December 2006. 2

3. Layered Double Hydroxides Ordered layered double hydroxides (LDHs) consisting of zinc and/or copper hydroxides were synthesized and combined with aluminum or gallium. These LDH compounds were then applied as photo catalysts to convert gaseous CO 2 (2.3 kPa) to methanol or CO. LDH were chosen with the expectation of 1)Sorption capacity for CO 2 in the layered space and 2)Tunable semiconductor properties (photo catalytically active) as a result of the choice of metal cations.

4. Catalysts Prepared 1)Zn-Cu-Al {[Zn 1.5 Cu 1.5 Al(OH) 8 ] 2 + (CO 3 ) 2- } 2)Zn-Cu-Ga {[Zn 1.5 Cu 1.5 Al(OH) 8 ] 2 + (CO 3 ) 2- } 3)Zn-Cu-In {[Zn 1.5 Cu 1.5 Al(OH) 8 ] 2 + (CO 3 ) 2- } 4)[Zn-Ni]-Cu-Al {[Zn 1.0 Ni 0.5 Cu 1.5 Al(OH) 8 ] 2 + (CO 3 ) 2- } 5)Zn-Cu-[Al-In] {{[Zn 1.5 Cu 1.5 In 0.3 Al 0.7 (OH) 8 ] 2 + (CO 3 ) 2- } 6)Zn-Cu-[Al-Fe] {[Zn 1.5 Cu 1.5 Fe 0.2 Al 0.8 (OH) 8 ] 2 + (CO 3 ) 2- } 7)CuO(3%) loaded on TiO 2 (P-25) 8)Ag(7%) loaded on TiO 2 (P-25)

5. Zn Cu Al ([Zn1.5 Cu 1.5 Al (OH)8] 2 +(CO3) 2 -. mH2O ) 100ml of Na2CO3 1M of NaOH to maintain P H 8(20ml) Zinc nitrate hexahydrate(0.375M), Copper nitrate trihydrate(0.375M),Alum inum nitrate nonahydrate(0.25M) of each 20ml Stirred at 290K for 2 hrs Stirred at 353K for 22hrs Filtered, Washed & Dried at ambient temp of 5 days Synthesis of Zn-Cu-Al LDH:

6. All Characterizations for Zn-Cu-Al Catalyst.

7. XRD pattern of Zn Cu Al according the literature: A.L.Patterson, Physical Review, 56 (1939).

8. All Characterizations for Zn-Cu-Ga Catalyst.

9. All Characterizations for Zn-Cu-In Catalyst.

10. Crystallite Size The crystallite size of the Zn Cu Al catalyst was calculated by the Scherrer’s formula: t = k*λ β* Cos θ Where t = crystallite size K = constant dependent on crystallite shape (0.9) λ = x-ray wavelength (1.54056 Å) β = FWHM (full width at half maximum) θ = Bragg’s angle

11. CRYSTALLITE SIZE & BAND GAP VALUES FOR HYDROTALCITES. Sample XRD Crystallite Size(nm) UV-Vis Band Gap(eV) Zn Cu Al20.44.1 Zn Cu Ga25.43.5 Zn Cu In21.23.1

12. TEXTURAL PROPERTIES OF HYDROTALCITES Sample Surface area m 2 /g Pore vol. cm 3 /g Pore dia. Å Zn Cu Al25.60.189305.0 Zn Cu Ga47.10.186127.9 Zn Cu In72.80.394202.6

13. [Zn-Ni]-Cu-Al 100ml of Na2CO3 1M of NaOH to maintain P H 8(20ml) Zinc nitrate hexahydrate(0.25M), Nickel nitrate (0.125M),Copper nitrate trihydrate(0.375M),Alum inum nitrate nonahydrate(0.25M) of each 20ml Stirred at 290K for 2 hrs Stirred at 353K for 22hrs Filtered, Washed & Dried at ambient temp of 5 days Synthesis of Zn&Ni-Cu-Al LDH:

14. All Characterizations for [Zn-Ni]-Cu-Al.

15. All Characterizations for Zn-Cu-[Al-In]

16. All Characterizations for Zn-Cu-[Al-Fe].

17. CRYSTALLITE SIZE & BAND GAP VALUES FOR HYDROTALCITES. Sample XRD Crystallite Size(nm) UV-Vis Band Gap(eV) Zn&Ni Cu Al12.63.3 Zn Cu Al&In25.73.2 Zn Cu Al&Fe46.41.9

18. Photo reduction of CO 2 : The Photo catalytic reduction was carried out in a glass reactor with a window through which CO 2 medium was irradiated. Catalyst (0.4 g) and 400 ml of 1M NaOH was added to the reactor. After bubbling CO 2 for 30 min, the reaction medium was irradiated. The reactor was tightly closed during the reaction. The solution in the reactor was continuously stirred by a magnetic stirrer during irradiation by 250 W Hg lamp. Reactions were carried out using Zn Cu Al.

19. Schematic Representation of Photo Catalytic Reactor for Liquid Phase

20. Reaction Conditions Reactor volume – 620 ml. Reaction medium – 400 ml of 0.2 N NaOH. Catalyst loaded – 0.4 g. CO2 was bubbled for 30 min. Reaction medium was irradiated through a 5 cm diameter quartz window. Hg lamp with 77 W power was used.

21. Reaction Conditions The products were analyzed using Clarus 500 Perkin Elmer Gas chromatography using Poroplot Q, 30 m and the detector is FID. Injector Temperature: 250°C Column Temperature: 150°C isothermal, Hold – 20 min Detector Temperature: 250°C Carrier Gas: Nitrogen, 1.5 ml/min. During the reaction, gas samples (0.3 ml) were withdrawn for every ONE hour from the reactor and injected into the GC.

22. Schematic Representation of Photo Catalytic Reactor for Gas Phase

23. CO 2 Photo Reduction for Zn Cu Al in Liquid Phase Time Methane (micro mole) Ethane (micro mole) Methanol (micro mole) Acetaldehyde (micro mole) Ethanol (micro mole) 000000 10.40.021200166 200.5950.0117 38.630013 400.01530.5541 500.157108 600.01000 700.043000 800.023809.2

24. CO 2 Photo Reduction for Zn Cu Al in Gas Phase Time Methane (micro mole) Ethane (micro mole) Methanol (micro mole) Acetaldehyde (micro mole) Ethanol (micro mole) 000000 10.10.00830120 20.150.0135160 300020 400020 50002.60.8 600060 70.10.00246.5100 80.40.0157150

25. CO 2 Photo Reduction for Zn Cu Al in both Liquid & Vapor Phase

26. Evaluation of ALL catalysts in vapour phase (methanol, methane, Ethane, Acetaldehyde)

27. Based on the product patterns observed for the six catalysts, the order of activity for methanol formation increases in the following order: Zn-Cu--In] < Zn-Cu-[Al-In] < Zn-Cu-[Al-Fe] < Zn-Cu-Al< Zn-Cu-Ga < [Zn-Ni] -Cu-Al The following are the significant outcome of the investigation at this stage: Substitution of the divalent Zn by Ni brings about substantial increase in the overall activity for CO 2 photoreduction. Substantial changes in the band gap values observed for various hydrotalcite samples could be one of the reasons for the observed differences in the activity. Shifting of light absorption edge towards visible range would increase the activity with visible light. In the case of [Zn-Ni] –Cu-Al very small particle size could be an additional factor for the observed increase in activity. For many photo catalysts based on perovskite structure, NiO is identified as highly efficient co-catalyst. It is worth exploring the whole range of compositions from Zn-Cu-Al to Ni-Cu-Al and also the incorporation of Ni in Zn-Cu- Ga system. Though the band gap for Zn-Cu-[Al-Fe] is less its amorphous nature and larger crystallite size may adversely affect the activity. In the In based hydrotalcites, presence of free In(OH) 3 as additional phase as observed in XRD patterns could affect the performance.

28. Design of Solar Reactor  A Photo Chemical solar reactor is a device to test the activity of various catalysts for reduction of carbon dioxide and water under sunlight.  The present study describes a solar reactor for carrying out photochemical energy storage, by the photosynthetic conversion of carbon dioxide and water over photoactive materials into useful organic compounds, particularly methanol, ethane etc.  The purpose of testing these reactions in actual sunlight is to obtain direct values of solar to chemical energy conversion efficiencies.

29. Design of SOLAR REACTOR

30. Solar Reactor

31. Preparation of CuO(3 %) loaded TiO2 (P25) catalyst CuO(3%) loaded TiO 2 (P25) catalyst was prepared by impregnating copper nitrate solution on P25 catalyst. Appropriate amount of P25 and copper nitrate solution was taken together and sonicated for 1 h and then stirred for 4 hrs at 95°C. It was then dried at 150°C for 2hrs at a ramp rate of 3°C/min. The dried sample was then calcined at 500°C for 0.5 h at the ramp rate of 5°C/min to obtain CuO(3 %) loaded TiO 2.

32. Evaluation of photo catalysts (P-25, 7% Ag/P-25 and 3% CuO/P-25) using solar reactor

33. Conclusion  It was of interest to explore Hydrotalcite use as photo catalysts for a number of conversions, especially for photo reduction of CO 2 with water.  Methanol, ethanol and acetaldehyde were the major products observed in all cases. Though methane and ethane were identified they could not be quantified.  Methanol and ethanol with very little amount of acetaldehyde are formed in aqueous phase, while only methanol and acetaldehyde are observed in vapour phase. Ethanol is not observed in vapour phase. Besides, the total amount of hydrocarbons formed in aqueous phase is higher than that in vapour phase.  In aqueous phase availability of water in excess ensures higher hydrogen production by water splitting and hence reduction of CO 2 could proceed upto ethanol stage.  In the vapour phase CO 2 is in excess and availability of hydrogen is less and hence reduction stops at acetaldehyde stage. For the same reason, overall conversion in aqueous phase is higher.  In the case of [Zn-Ni] –Cu-Al very small particle size could be an additional factor for the observed increase in activity.

34.  Of all the catalysts, [Zn-Ni]-Cu-Al system displayed maximum activity and good stability in vapour phase reaction, yielding ~300 micro moles of methanol/ gram of the catalyst.  Substitution of the divalent Zn by Ni brings about substantial increase in the overall activity for CO 2 photo reduction.  For many photo catalysts based on perovskite/pyrochlore structure, NiO is identified as highly efficient co-catalyst. The mechanism of promotion by Ni in this case is to be investigated further.  in the present case hydrogen generated in-situ by photo catalytic water splitting is used. Due to this reason activity observed in the present case is higher, in micromoles/g compared to nano moles /g observed earlier.  Carrying out the reaction with suitable CO 2 : H 2 mole ratio by increasing the partial pressure of water (or decreasing that of CO 2 ) could help to increase conversion and improve catalyst stability.  compositional changes in terms of substituting divalent and trivalent ions could lead to a number of potentially active catalysts that may display higher activity.

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