Computational and Experimental Studies of CO 2 Photocatalytic Reduction on Undoped and Lanthanide-doped Titania

1. ENERGY INSTITUTE, DEPARTMENT OF ENERGY AND MINERAL ENGINEERING Computational and Experimental Studies of CO2 Photocatalytic Reduction on Undoped and Lanthanide-doped Titania Presented by: Venkata Pradeep Indrakanti Co-authors: J.D.Kubicki, M.M. Maroto-Valer, H.H. Schobert July 11, 2007 CHEMRAWN-XVII and ICCDU-IX conference on greenhouse gases

2. Overview • Introduction • Background & Motivation • Objectives • Methodology • Results & Discussion • Conclusions & Future Work

3. Routes to CO2 utilization CHEMICALS FUELS CH3OH, CH4, CO Catalytic hydrogenation Electrochemical reduction Urea, cyclic carbonates, polycarbonates, polyurethane intermediates etc. Photochemical reduction HOMOGENEOUS SYSTEMS HETEROGENEOUS SYSTEMS

4. CO2 photocatalytic reduction

5. Brief background & Motivation • Background • First study : Inoue, Fujishima et al. (1979) • Notable recent advances: • Ti on various supports (Anpo and coworkers) • Binuclear (Zr4+-Sn2+) redox catalysts (Frei and coworkers) • How does heterogeneous CO2 photocatalytic production of CH4 vs H2 from solar water photo(electro)lysis ? • ~ 1mmol/h of CH4 (Anpo et al., (1998)) vs 400-700 mmol/h of H2 (Aroutiounian et al.,(2005)) • Lanthanide doping • Reduced band gap • Increased charge carrier lifetimes (Xu et al. (2002)) • Motivation • Understand initial steps of CO2 photoreduction on TiO2 surfaces • Study effect of lanthanide doping on CO2 photoreduction

6. Objectives • Identify the intermediate species formed during CO2 photoreduction by modeling the excited-state chemistry of CO2 species adsorbed on various low index surface planes of TiO2 . • Test the hypothesis of dopant-induced charge carrier trapping affecting photoactivity by performing photoreactions with various concentrations of La, Sm and Gd-doped TiO2 as photocatalysts. • Study the intermediates of this photoreduction through spectroscopic characterizations and use computational methods to corroborate/complement spectroscopic data.

7. Computational Methods • Software ? • Gaussian 03, Rev. D.01 • http://gears.aset.psu.edu/hpc/systems/ • What surface plane? • (010), (001), (101) • What calculation method/basis set? • B3LYP/6-31+G(d) • CASSCF /SAC-CI for excited states • Typical modeling procedure: • Prepare 2 atom Ti cluster from appropriate face • Model CO2 adsorption on ground state rigid cluster • Relax cluster and optimize, compare n s to experiment • Model lowest triplet state, compare q, n and spin r

8. Optimized Ti2O9H10CO2 (010) singletB3LYP/6-31+G(d), C2 symmetry Bond lengths in Å, angles in degrees

9. Comparison of experimental (bi)carbonate IR frequencies with ground state B3LYP/6-31G(d) calculations

10. O: 0.42 O: 0.57 Atomic spin density on Ti: 1.04 Lowest excited Ti2O9H10CO2 (010) tripletB3LYP/6-31+G(d) CO3 species (triplet-singlet dq) = -0.29 e- Bond lengths in Å, angles in degrees

11. Excited triplet (010) C-O IR frequencies

12. HOMO LUMO Molecular orbitals of (010) Ti2O9H10CO2B3LYP/6-31+G(d) (ground state singlet) Also MO # 73

13. Fate of CO3•d- • Chandrasekharan and Thomas,1983 • CO3•- radicals, flash photolysis of Na2CO3 + anatase slurry • CO3→ CO + O2 • Scheerer and Graetzel,1976 • CO3→ CO2 + ½ O2 • Getoff,1994 • Coupling reactions forming reduced products • Emeline et al.,2005 • CO3•- reacts with H2 to yield reduced products • No experimental verification for CO3→ CO + O2

14. Conclusions and Future Work : Computations • Conclusions • First study to model CO3•--like radicals on anatase (010) surfaces • O-Ti charge transfer vs. reaction of CO32- with OH• radicals • Relevant intermediate? • Future Work • DFT results : guide to post-HF methods for excited state geometry optimizations • Addition of water

15. Experimental Methods • La-, Gd-, Sm- doped TiO2 • Sol-gel synthesis • TiO2 coating • Photoreactions • EPR experiments on promising catalysts • CW and FT-EPR • Identify electron/hole trapping sites, CO2.-, CO3.-..

16. Photocatalysts tested

17. Photoreactor Setup

18. Typical CO2 photoreduction results

19. Pulsed EPR spectra of sol-gel TiO2 Intensity (arbitrary units)

20. Conclusions and Future Work: Experiments • Conclusions: • La doped TiO2 : CO2→ CH4 • Color change of irradiated TiO2 can be tracked with pulsed EPR • Future Work • Quantitatively estimate CH4 • Test Sm- and Gd-doped TiO2 • Pulsed EPR work at higher temperatures (77 K)

21. References • Inoue, T., A. Fujishima, et al. (1979). "Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders." Nature (London, United Kingdom) 277(5698): 637-8. • Aroutiounian, V. M., V. M. Arakelyan, et al. (2005). "Metal oxide photoelectrodes for hydrogen generation using solar radiation-driven water splitting." Solar Energy 78(5): 581-592. • Xu, A.-W., Y. Gao, et al. (2002). "The preparation, characterization, and their photocatalytic activities of rare-earth-doped TiO2 nanoparticles." JOURNAL OF CATALYSIS 207(2): 151-157. • Ramis, G., G. Busca and V. Lorenzelli (1991). Low-temperature carbon dioxide adsorption on metal oxides: spectroscopic characterization of some weakly adsorbed species. Materials Chemistry and Physics 29(1-4): 425-435. • Martra, G. (2000). Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour. Applied Catalysis, A: General 200(1-2): 275-285. • Jacox, M. E. and D. E. Milligan (1974). Matrix-Isolation Study Of Vibrational-Spectrum And Structure Of Co3-Radical Anion. Journal of Molecular Spectroscopy 52(3): 363-379. • Chandrasekaran, K. and J. K. Thomas (1983). Photochemical reduction of carbonate to formaldehyde on titanium dioxide powder. Chemical Physics Letters 99(1): 7-1a0. • Scheerer, R. and M. Graetzel (1976). Photo-induced oxidation of carbonate ions by duroquinone, a pathway of oxygen evolution from water by visible light. Berichte der Bunsen-Gesellschaft 80(10): 979-982. • Emeline, A. V., G. V. Kataeva, A. V. Panasuk, V. K. Ryabchuk, N. V. Sheremetyeva and N. Serpone (2005). Effect of surface photoreactions on the photocoloration of a wide band gap metal oxide: Probing whether surface reactions are photocatalytic. Journal of Physical Chemistry B 109(11): 5175-5185. • Getoff, N. (1994). Possibilities on the radiation-induced incorporation of CO2 and CO into organic compounds. International Journal of Hydrogen Energy 19(8): 667-672. • Froese, R. D. J. and J. D. Goddard (1993). Features Of The Lowest Singlet And Triplet Potential-Energy Surfaces Of CO3. Journal of Physical Chemistry 97(29): 7484-7490.

22. Acknowledgments • Penn State Institutes of Energy and Environment (PSIEE) & Materials Research Institute, Penn State • EMS Centennial Research Travel Award, College of Earth and Mineral Sciences, Penn State • Dr. Alexander Mitin, Penn State • Dr. LiPuma, SChEME, University of Nottingham

23. Questions