Co-Doped TiO2 Aerogel with Enhanced Light Absorption Properties for CO2 Photocatalytic Reduction and H2o Splitting

Abstract

Carbon dioxide (CO2) is a major component of greenhouse gases, whose amount in the atmosphere is gradually increasing. The use of various methods to reduce CO2 has become a hot research topic. However, CO2 being chemically stable, traditional methods such as thermal ones are not efficient. Artificial photosynthesis may alternatively reduce CO2 under mild conditions. Therefore, the development of innovative photocatalysts to produce high-value-added products from CO2 reduction has attracted wide interest from academia and industry, providing new insights into the utilization of CO2. However, the activation of CO2 remains difficult, the reaction path is complex and the selectivity is low. Therefore, the design of photocatalysts with both high selectivity and conversion rate for CO2 reduction is of great scientific and strategic importance.1 TiO2 is a common photocatalyst, but pure TiO2 is only responsive to UV light. Hence, modification of TiO2 by doping or co-catalyst loading has been widely studied to enhance its response to visible light. Aerogels have a high specific surface area and a tunable porosity, both features which are very suitable for catalysis. In the case of CO2 reduction, high specific surface area will favor CO2 adsorption, which is beneficial for the following activation steps.Therefore, based on the previous work on aerogels in our group,2 a series of TiO2 aerogels doped with niobium and nitrogen were synthesized, and Cu was deposited at the surface of the aerogel as a co-catalyst to help CO2 reduction.Obtained photocatalysts were characterized to determine their crystallographic structure (XRD), composition (EDX, XPS), morphology (SEM, N2 sorption) and optical properties (UV-Vis absorption). After calcination, the anatase phase of TiO2 was detected, without any heterogeneous phase generated after Nb and N doping. Nb doping made the anatase (101) peak blue-shifted due to the pinning effect, and the maximum possible doping amount of Nb was 10 at. % in our case. Moreover, no CuxO species were observed in the XRD patterns of Cu/TiO2, which tentatively indicates that it is uniformly distributed as nanoscale particles or domains.The specific surface area (SBET) obtained from N2 sorption experiments was higher for TiO2 aerogel (72 m2/g) than for commercial catalyst P25 (50 m2/g). After Nb doping, the particle size of TiO2 decreased (statistics from SEM image) and as a consequence, SBET was significantly increased (123 m2/g for 10 at. % of Nb).The light absorption ability of TiO2 aerogels was notably shifted in the visible light region after doping, resulting in a narrower bandgap. The visible light absorption performance was even more enhanced after cupper deposition, maybe due to localized Surface Plasmon Resonance effect (LSPR) of Cu2+.The photocatalytic performance was evaluated in a homemade photocatalytic reactor, illuminated with a Xe-light source simulating sunlight and equipped with a monochromator and an optical power meter to investigate the mechanism of photocatalytic reactions.Cu/TiO2: Nb, N aerogel photocatalysts were tested for CO2 photocatalytic reduction, and promising methanol yields were obtained. Parameters such as temperature and pressure and feed ratios will be adjusted to optimize selectivity and conversion rate. The prepared catalysts will also be evaluated for hydrogen production by water splitting. Reference: O. Ola and M. M. Maroto-Valer, J. Photochem. Photobiol. C Photochem. Rev., 24, 16–42 (2015).C. Beauger, L. Testut, S. Berthon-Fabry, F. Georgi, and L. Guetaz, Microporous Mesoporous Mater., 232, 109–118 (2016). Figure 1