No abstract

The conversion of CO2 and water into value-added fuels with visible light is difficult to achieve in inorganic photocatalytic systems. However, we synthesized a ternary catalyst, CdS/(Cu-TNTs), which is assembled on a core of sodium trititanate nanotubes (TNTs; NaxH2-xTi3O7) decorated with elemental copper deposits followed by an overcoat of CdS quantum dot deposits. This ternary photocatalyst is capable of catalyzing the conversion of CO2 and water into C1-C3 hydrocarbons (e.g., CH4, C2H6, C3H8, C2H4, C3H6) upon irradiation with visible light above 420 nm. With this composite photocatalyst, sacrificial electron donors are not required for the photoreduction of CO2. We have shown that water is the principal photoexcited-state electron donor, while CO2 bound to the composite surface serves as the corresponding electron acceptor. If the photochemical reaction is carried out under an atmosphere of 99.9% (13)CO2, then the product hydrocarbons are built upon a (13)C backbone. However, free molecular H2 is not observed over 5 h of visible light irradiation even though proton reduction in aqueous solution is thermodynamically favored over CO2 reduction. In terms of photocatalytic efficiency, the stoichiometric fraction of Na(+) in TNTs appears to be an important factor that influences the formation of the observed hydrocarbons. The coordination of CO2 to surface exchange sites on the ternary catalyst leads to the formation of surface-bound CO2 and related carbonate species. It appears that the bidentate binding of O═C═O to certain reactive surface sites reduces the energy barrier for conduction band electron transfer to CO2. The methyl radical (CH3(•)), an observed intermediate in the reaction, was positively identified using an ESR spin trapping probe molecule. The copper deposits on the surface of TNTs appear to play a major role in the transient trapping of methyl radical, which in turn self-reacts to produce ethane.

Characterizations of microwave-induced titanate nanotubes (NaxH(2-x)Ti3O7, TNTs) were conducted by the determinations of specific surface area (S(BET)), X-ray diffraction (XRD), X-ray photoelectron spectroscopic (XPS), ionic coupled plasma-atomic emission spectrometry(ICP-AES), scanning electron microscopy/ energy dispersive X-ray (SEM/EDX), and high-resolution transmission electron microscopy (HR-TEM). The applied level of microwave irradiation during the fabrication process is responsible for both the intercalation intensity of Na atoms into TNTs and the type of crystallization phase within TNTs, which dominate the efficiency of photocatalytic NH3/NH4+. A pure TNT phase presents no powerful ability toward photocatalytic NH3/ NH4+, while the photocatalytic efficiency can be enhanced with the presence of a rutile phase within TNTs. In addition, the mixture of anatase and rutile phase within P25 TiO2 prefers forming NO3-, whereas TNTs yield higher NO2- amount Regarding the effect of acid-washing treatment on TNTs, the acid-treated TNTs with enhanced ion exchangeability considerably improve the NH3/NH4+ degradation and NO2-/NO3- yields. This result is likely ascribed to the easy intercalation of NH3/ NH4+ into the structure of acid-washing TNTs so that the photocatalytic oxidation of intercalated NH3/NH4+ is not limited to the shielding effect resulting from the overload of TNTs.

This study aimed to characterize N-doped TiO2 prepared by thermally treating microwave-assisted titanate nanotubes (TNTs, Na x H2−x Ti3O7) in an Ar/NH3 atmosphere. The effect of intercalated Na(I) within TNTs on the visible light photoactivity and the N-doping mode was investigated as well. By evaluating the performance of photocatalytic oxidation of phenol under the visible region, the photoactivity of N-doped TiO2 prepared from TNTs is 3 times higher than that of N-doped TiO2 prepared from P25 TiO2. Characterizations, including HR-TEM, XRD, XPS, NH3-TPD, UV−vis DRS, and S BET, indicate that the substitutional N-doping mode, the O−Ti−N linkage, is mainly responsible for narrowing the band gap and eventually enhancing the visible light photoactivity. Furthermore, the doping mechanism is significantly dependent on the presence of intercalated Na(I) within TNTs. The O−Ti−N linkage, owing to the substitutional doping, is apparent for TNTs with a low content of intercalated Na(I), whereas the presence of the higher amount of intercalated Na(I) leads to the formation of the Ti−N−O linkage that is considered as an interstitial doping mode. Also, the presence of intercalated Na(I) during the doping process results in the formation of Na2Ti6O13 instead of an inert TiN crystallinity, which is advantageous to enhancing the photoactivity of N-doped TiO2 due to the effect of interphase electron transfer between Na2Ti6O13 and TiO2.