Thanaree Phongamwong, Metta Chareonpanich*, Jumras Limtrakul
The combination of TiO2 nanoparticles and chlorophyll molecules is promising to provide potential new composite materials for improving CO2 photoreduction. In this research, chlorophyll in Spirulina modified on N-doped TiO2 catalysts (Sp/N-TiO2) were synthesized with an attempt to enhance the photocatalytic efficiency. The effects of nitrogen and chlorophyll in Spirulina on CO2 photoreduction with water under visible light of Sp/N-TiO2 were investigated by using the experimental system shown in Fig. 1
Fig. 1 Schematic of experimental system for CO2 photoreduction.
Fig. 2 Normalized Ti K-edge XANES spectra of (a) Fig. 3 High resolution XPS spectra of (A) N 1s, (B) Ti 2p of modified
anatase, (b) rutile, (c) TiO2, and (d)–(i) modified TiO2. TiO2 catalysts, and (C) survey spectrum of 0.5Sp/10N-TiO2 catalyst.
As shown in Fig. 2, the normalized Ti K-edge XANES spectra, carried out at BL8 of the SLRI, evidently confirmed the existence of anatase phase of all catalysts. N 1S XPS spectra in Fig. 3, carried out at BL3.2a of the SLRI, confirmed the present of nitrogen dopants in the form of interstitial N in TiO2 lattice which were responsible for the increase in visible light activity of the N-TiO2 catalysts. It was found that approximately 2.91 at.% of nitrogen was successfully doped onto the 10N-TiO2 catalyst. In addition, Ti 2p XPS spectra exhibited an occurrence of Ti3+ oxidation state and oxygen vacancies (VO) on the surface of TiO2 catalysts acted as the active adsorption sites of reactants and intermediates. The survey spectrum of 0.5Sp/10N-TiO2 also exhibited the expose of Spirulina on the catalysts to light irradiation.
Fig. 4 Proposed mechanism for photoreduction of CO2 with water on Spirulina-modified N-TiO2 catalyst.
Regarding the catalytic performances of CO2 photoreduction, the activities of catalysts were in the order of undoped TiO2 < N-TiO2 < Sp/N-TiO2. The outstanding photocatalytic activity of Sp/N-TiO2 could be attributed to the enhancement in visible light harvesting, surface VO, and the synergistic effect between N-TiO2 and chlorophyll in Spirulina. Nevertheless, the addition of Spirulina was found to improve the catalytic stability and C2+ (C2H4 and C2H6) product selectivity. Accordingly, the possible reaction mechanism of CO2 reduction for the formation of hydrogen and hydrocarbon products in this present work is illustrated in Fig. 4.
Source: T. Phongamwong, M. Chareonpanich, J. Limtrakul, Applied Catalysis B: Environmental 168 (2015) 114–124.