Selective photocatalytic reduction of CO2 to methanol in CuO-loaded NaTaO3 nanocubes in isopropanol.

Selective photocatalytic reduction of CO2 to methanol in CuO-loaded NaTaO3 nanocubes in isopropanol Tianyu Xiang1,2, Feng Xin*1,2,§, Jingshuai Chen1,2, Yuwen Wang1,2, Xiaohong Yin3 and Xiao Shao3 Full Research Paper Open Access Address: 1School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China, 2Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China and 3School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China Beilstein J. Nanotechnol. 2016, 7, 776–783. doi:10.3762/bjnano.7.69 Email: Feng Xin* - Associate Editor: R. Xu * Corresponding author § Tel.: +86 22 27409533; Fax: +86 22 27892359 Received: 10 March 2016 Accepted: 12 May 2016 Published: 01 June 2016 © 2016 Xiang et al; licensee Beilstein-Institut. License and terms: see end of document. Keywords: CO2 reduction; CuO loading; isopropanol; NaTaO3 nanocubes; photocatalysis Abstract A series of NaTaO3 photocatalysts were prepared with Ta2O5 and NaOH via a hydrothermal method. CuO was loaded onto the surface of NaTaO3 as a cocatalyst by successive impregnation and calcination. The obtained photocatalysts were characterized by XRD, SEM, UV–vis, EDS and XPS and used to photocatalytically reduce CO2 in isopropanol. This worked to both absorb CO2 and as a sacrificial reagent to harvest CO2 and donate electrons. Methanol and acetone were generated as the reduction product of CO2 and the oxidation product of isopropanol, respectively. NaTaO3 nanocubes loaded with 2 wt % CuO and synthesized in 2 mol/L NaOH solution showed the best activity. The methanol and acetone yields were 137.48 μmol/(g·h) and 335.93 μmol/(g·h), respectively, after 6 h of irradiation. Such high activity could be attributed to the good crystallinity, morphology and proper amount of CuO loading, which functioned as reductive sites for selective formation of methanol. The reaction mechanism was also proposed and explained by band theory. Introduction Global warming is one of the most major environmental problems that we are facing in the 21st century [1]. Carbon dioxide (CO2) contributes significantly to global climate change as it is the main greenhouse gas present in the atmosphere and primarily formed from the consumption of fossil fuels [2]. To date, many methods have been proposed to reduce the emitted CO2 concentration. A particularly advantageous approach is the capture of CO2 from the atmosphere for the conversion to fuel by using a sustainable source of energy like sunlight. In this way, global warming and energy shortage problems can be solved simultaneously [3-7]. For this purpose, the photocatalytic conversion of CO2 to fuel is particularly emphasized. In 1979, Inoue et al. [8] first reported the photocatalytic reduction of CO2 in aqueous solution using several semiconductor materials (WO3, TiO2, ZnO, CdS, GaP and SiC), producing 776 Beilstein J. Nanotechnol. 2016, 7, 776–783. CH3OH, HCOOH, HCHO and trace amounts of CH4. In the 1990s, Ta oxide photocatalysts began to draw attention in the field of water splitting. A series of Ta catalysts, such as LiTaO3 [9], NaTaO3 [10], KTaO3 [11], AgTaO3 [12], CaTa2O6 [13], SrTa2O6 [13], KBa2Ta3O10 [14], were proved to efficiently split water. In the 21st century, the study of Ta catalysts for the reduction of CO2 began. Kentaro Teramura et al. [15] prepared ATaO3 (A = Li, Na, K) compounds using a solid state reaction (SSR) method to reduce CO2 in the presence of H2. The only product was CO and the order of photocatalytic activity was LiTaO3 > NaTaO3 > KTaO3, which was consistent with that of the Eg (band gap) values. However, the highest yield of CO in LiTaO3 was 0.42 μmol/g after 24 h of photoirradiation, which was still far from satisfactory. Ye et al. [16] synthesized a series of noble-metal-loaded NaTaO3 samples to reduce CO2 with water. H 2 was introduced into this process as an electron donor. Ru/NaTaO 3 was found to have the best activity (CH4 51.8 μmol/(g·h)) and product selectivity in converting CO2 to CH4. Junwang Tang and his team [17] prepared KTaO3 nanoflakes by a solvothermal method in a hexane–water mixture and reduced CO2 using pure water as an electron donor. The activity was quite high for both H2 and CO production, achieving 20× (H2) and 7× (CO) higher than that of the cubic sample prepared by the solid state reaction. This was an indication that the catalyst morphology played a crucial role in activity. Jeffrey C. S. Wu et al. [18] prepared NiO-loaded InTaO4 photocatalysts by a sol–gel method and carried out the photocatalytic reduction of CO2 in a self-made optical fiber reactor filled with 0.2 mol/L NaOH solution. The formation rate of methanol was 11.1 μmol/(g·h) under halogen lamp irradiation at 25 °C. Ru-Shi Liu and co-workers [19] prepared a series of nanostructured core–shell materials (Ni@NiO/N-doped InTaO4 photocatalysts) for the reduction of CO2 to methanol in pure water. In these structures, the core–shell nanostructure might offer a new reaction center transferred from the surface of the InTaO4 material. In this paper, we report the photocatalytic reduction of CO2 to methanol using CuO-loaded NaTaO 3 catalysts. NaTaO 3 nanocubes were synthesized via a hydrothermal method using Ta2O5 and NaOH. CuO was loaded onto the surface of NaTaO3 by impregnation, where CuO acts as a cocatalyst for CO2 reduction, promoting charge transfer and limiting the fast recombination of electrons and holes [20,21]. According to the literature, Cu oxides and Cu cations are active cocatalysts for CO2 reduction and could serve as reductive sites for selective reduction of CO2 to methanol [22-27]. Isopropanol was employed as both an absorber and a sacrificial reagent due to its good capability to absorb CO2 and donate electrons [28-30]. Acetone, an important industrial material, was generated as the oxidation product of isopropanol. Experimental Catalyst preparation Tantalum oxide (Ta2O5, 99.99%), sodium hydroxide (NaOH, 96%) and isopropanol (iPrOH, 99.9%) were purchased from Aladdin Industrial Corporation. Copper nitrate (Cu(NO3)2·3H2O, AR) was purchased from Tianjin Guangfu Chemical Reagent Company. All reagents were used as received without any further purification. The NaTaO3 nanocubes were synthesized by a hydrothermal method as reported by Li et al. [31]. In a typical procedure, 0.442 g of Ta2O5 and a sufficient amount of NaOH were added into a Teflon-lined autoclave with a total volume of 50 mL, and deionized water was filled up to 40 mL. The autoclave temperature was held at 140 °C for 12 h then cooled to room temperature in air. The obtained product was washed with deionized water several times before being dried at 80 °C in an oven overnight. The as-prepared catalysts were denoted as 1M-NaTaO3, 2M-NaTaO3, 3M-NaTaO3, 4M-NaTaO3, corresponding to a NaOH concentration of 1 mol/L, 2 mol/L, 3 mol/L, 4 mol/L, respectively. CuO was loaded onto the surface of NaTa (...truncated)