Copper-doped titania photocatalysts for simultaneous reduction of CO2 and production of H2 from aqueous sulfide

Abstract

Copper-doped titanium dioxide materials with anatase phase (Cu-TiO 2 , atomic Cu contents ranging from 0 to 3% relative to the sum of Cu and Ti), and particle sizes of 12-15 nm, were synthesised by a solvo-thermal method using ethanol as the solvent and small amounts of water to promote the hydrolysis-condensation processes. Diffuse reflectance UV-vis spectroscopy show that the edges of absorption of the titania materials are somewhat shifted to higher wavelengths due to the presence of Cu. X-Ray Photoelectron Spectroscopy (XPS) indicate that Cu(II) is predominant. Photocatalytic CO 2 reduction experiments were performed in aqueous Cu-TiO 2 suspensions under UV-rich light and in the presence of different solutes. Sulfide was found to promote the efficient production of H 2 from water and formic acid from CO 2 . The effect of the Cu content on the photoactivity of Cu-TiO 2 was also studied, showing that copper plays a role on the photocatalytic reduction of CO 2 . IntroductionCarbon dioxide is an inexpensive and widely available feedstock. Its use for the production of chemicals is doubly beneficial, since in addition to its availability, it would represent a decrease in the carbon footprint for the manufacturing of the particular chemical. Other researchers have reported lower but noticeable methanol yields using similarly prepared photocatalysts and Hg lamp irradiation (10-23 µmol g cat −1 h −1 ) [19,20]. In gas-phase reactions, the formation of methanol was also reported, but in considerably lower yields (2 nmol g cat −1 h −1 ), being methane the major product formed by Cu/TiO 2 photocatalysis [21]. In contrast, similar materials were found to promote the formation of several hydrocarbons including methane, ethane and ethylene in conjunction with larger amounts of H 2 [22], whereas CO and methane were major products found in a more recent study [23]. A significant degree of discrepancy regarding the identity of the reaction products (in addition to their selectivities and yields) is apparent by considering the aforementioned literature data, although this could be in part due to the different synthetic procedures used for the preparation of the photocatalysts and to the varied irradiation conditions used.Despite the notable progress in this field of research, there is no clear trend which may allow to conclude on the actual mechanistic route and to gain knowledge on the role of copper on the reaction. Thorough kinetic and in situ spectroscopic studies would provide valuable information on the reaction at the surface of the photocatalyst and may guide future design of more active copper-titania materials. An important issue to focus on is the possible back-reactions of the reduction products on the photocatalyst. For example, it was observed that methanol and formaldehyde, initially formed by a photocatalytic process using Cu(0) powder and TiO 2 , were 3 rapidly consumed by in situ formed Cu/TiO 2 [24]. The simultaneous occurrence of CO 2 reduction and decomposition of the products should result in stat...

Improving the photo‐reduction of CO2 to fuels with catalysts synthesized under high pressure: Cu/TiO2

“…Although some authors, suggest that increasing copper concentration promotes sintering process, increasing crystallite size and decreasing BET area, in view of the XRD results described below (no change in crystallinity with metal loading), we prefer to explain this trend in terms of a partial coverage of TiO 2 by metal species . Some works dealing with traditional synthesis of Cu/TiO 2 (solvo‐thermal) do not report any change in BET area with variable metal concentrations , …”

Dynamic photocatalytic reduction of CO 2 to CO in a honeycomb monolith reactor loaded with Cu and N doped TiO 2 nanocatalysts

“…%, a reduction of photoactivity was observed. This was probably due to loading of excess Cu over the TiO 2 surface (shielding effect), causing a decrease of TiO 2 surface active sites or perhaps due to recombination centers over the TiO 2 surface [4,32]. Therefore, 3 wt.…”

“…The TiO 2 crystallite size was gradually reduced with Cu and N loading. This was probably Cu and N presented over the TiO 2 structure prohibited crystal growth, resulting in reduced crystallite sizes [4,32].…”

“…All the peaks of TiO 2 and Cu/N loaded TiO 2 samples were identical, thus shift in peaks were not observed. The peaks attributable to Cu or N in metal or oxide states were not detected, possibly due to the low amount, or they were uniformly distributed inside the TiO 2 structure [32,33]. The crystallite sizes were estimated from the XRD patterns using Scherer equation and the data is presented in Table 1.…”

ETS‑10 Modified with CuxO Nanoparticles and Their Application for the Conversion of CO2 and Water into Oxygenates

“…41,42 Following this trend, copper has received attention as a metal with activity for the reduction of CO 2 into methanol. 43,44 Both Cu 2 O and CuO are known to have suitable band gap energies, they are Earth-abundant and non-toxic materials. 45 They have been widely studied because they promote a significantly increase in the generation of C1-C3 hydrocarbons and oxygenated products.…”