Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4

Efficient catalysts with high selectivity in products are highly desirable for photocatalytic CO2 reduction. In this work, hydroxyapatite (HAP) decorated TiO2 (HAP/TiO2) were successfully fabricated via in-situ deposition of Ca(OH)(2) on rutile TiO2 followed by a facile hydrothermal reaction. Comparing with TiO2, HAP/TiO2 exhibited significant enhancement (ca. 40 times) toward photocatalytic CO2 reduction in the presence of H2O with a >95% selectivity of CH4. The characterizations revealed HAP possessed Lewis basic sites (O2- in -PO43- groups) and Lewis acidic sites (Ca2+ or OH- vacancies), where Lewis basic sites could enhance the adsorption/activation of CO2 and Lewis acidic sites facilitated the adsorption/dissociation of H2O respectively, thus promoting the photocatalytic reduction and oxidation half-reactions of CO2 and H2O over Pt/TiO2. The formation of much more stable intermediates over HAP/TiO2 would be responsible for the high selectivity of CH4. Moreover, photoelectrochemical and electrochemical characterizations revealed HAP could also promote the charge separation of TiO2 and the charge transfer between TiO2 and adsorbed species. The findings demonstrate HAP has a great potential as efficient assistant for photocatalytic CO2 reduction with H2O and will stimulate us to design novel semiconductor-based materials with tuned Lewis acidic and Lewis basic sites to achieve highly efficient photocatalysts. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.