Читать книгу Solar-to-Chemical Conversion - Группа авторов - Страница 24

Apart from the hydrocarbons, other carbon‐containing fuels such as CO also are produced as main product in CO₂‐involving photocatalysis [66]. From the view of redox potential, CO formation reaction (−0.48 eV vs. NHE) is inferior to the production of CH₄ (−0.38 eV vs. NHE) and CH₃OH (−0.24 eV vs. NHE), while the formation reactions of CH₄ and CH₃OH involve six‐ and eight‐electron reaction process, in which a series of elemental reaction are unclear and slow. In contrast, carbon monoxide is simpler than CH₄, CH₃OH, and other hydrocarbons, which means fewer needs of reductive electrons. Therefore, the CO reduction reaction is thermodynamically triggered, and CO is a preferred product in CO₂ photoreduction. For instance, Miyauchi and coworkers reported CuO‐decorated Nb₃O₈ nanosheets for photocatalytic CO₂ reduced into CO, and simultaneously the reaction pathway over this system had been deeply investigated through electron spin resonance (ESR) and isotope‐labeled experiments [67]. The results indicate that amorphous copper oxide nanoclusters can work as efficient electrocatalysts grafted onto the surface of niobate nanosheets for the reduction of carbon dioxide to carbon monoxide. Furthermore, the photocatalytic activity and reaction pathway of Cu(II)‐grafted Nb₃O₈ nanosheets were investigated using ESR analysis and isotope‐labeled molecules (H₂¹⁸O and ¹³CO₂). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen (¹⁸O₂) and then reduce CO₂ to produce ¹³CO. ESR analysis confirmed that excited holes in the VB of Nb₃O₈ nanosheets react with water and that excited electrons in the CB of Nb₃O₈ nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO₂ into CO. The Cu(II) nanocluster‐grafted Nb₃O₈ nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO₂ photoreduction systems. Furthermore, Tahir et al. synthesized Ag nanoparticles/TiO₂ nanowires core–shell heterojunction for efficient photoreduction of CO₂ to CO in the presence of hydrogen [68]. Ag‐NPs coated over TiO₂ NWs exhibited strong absorption of visible light due to localized surface plasmon resonance (LSPR) excitation, trapped electrons, and hindered charge recombination rate. The synergistic effect of Ag‐NPs coated over TiO₂ NWs for CO₂ conversion was evaluated in a gas‐phase system under UV and visible‐light irradiation. The plasmonic Ag‐NPs/TiO₂ NWs demonstrated excellent photoactivity in the reduction of CO₂ into CO, CH₄, and CH₃OH under visible‐light irradiation. The results show that 3 wt% Ag‐NPs‐loaded TiO₂ NWs was found to be the most active, giving the highest CO evolution of 983 mmol g⁻¹ h⁻¹ at selectivity 98%. This amount of CO produced was 23 times more than the TiO₂ NWs and 109 times larger than the yield of CO produced over the pure TiO₂. More importantly, the quantum yield was substantially enhanced for CO evolution. The LSPR excitation and synergic effect of Ag‐NPs that can effectively accelerate the charge separation were proposed to be responsible for the enhancement of photocatalytic activity. The photostability of Ag‐NPs/TiO₂ NWs evidenced in cyclic runs for selective CO production under visible light, yet photoactivity declined over the irradiation time under UV light.

Recently, it is well demonstrated that layered nanomaterials with excellent conductivity, e.g. graphene and C₃N₄, can facilitate to accelerate the charge transportation, which is a promising component to efficiently improve the separation efficiency of photogenerated electrons and holes in traditional semiconducting photocatalysts [69]. For instance, Ye and coworkers constructed a smooth carrier channel in the basal plane of organic polymeric C₃N₄ photocatalyst by enriching defects with short and strong C–N chains through a glycine linker so that the photoinduced carrier transfer is apparently enhanced. Based on this, the photoreduction ability of CO is increased by 29.2 times under a simulated sunlight irradiation, as shown in Figure 2.13 [70]. Compared with complicated hydrocarbons, H₂ has strong competition ability to CO due to the lower potential. Therefore, H₂ often consumes more photoexcited electrons and suppresses the CO production. Although H₂ is cleaner energy storage form, carbon element is not contained in the energy cycle, which is undesirable to control CO₂ level in atmosphere. In other words, as a competitive reaction, two‐electron H₂ evolution reaction from water splitting should be suppressed deliberately. In 2011, Kudo and coworkers reported a series of Ag‐modified ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) photocatalysts that have 3.79–3.85 eV of band gaps and layered perovskite structures, displaying photocatalytic activities of CO₂ reduction to produce CO and HCOOH without any sacrificial reagents [71]. It is reported that d⁰‐type metal oxide photocatalysts can catalyze water splitting into H₂ and O₂ because of high CB positions and wide band gaps, which are expected to be active for CO₂ photoreduction at a suitable reaction sites on the surface of photocatalysts. Among these photocatalysts, Ag‐loaded BaLa₄Ti₄O₁₅ showed the best photocatalytic activity. The Ag as a cocatalyst prepared by the liquid‐phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa₄Ti₄O₁₅. On the optimized Ag/BaLa₄Ti₄O₁₅ photocatalyst, CO was the main reduction product rather than H₂ even in an aqueous medium. Furthermore, evolution of O₂ in a stoichiometric ratio (H₂ + CO:O₂ = 2 : 1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO₂ reduction, which demonstrated that CO₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa₄Ti₄O₁₅ photocatalyst. After that, Teramura and coworkers designed a new three‐component heterojunction for efficiently hampering the H₂ evolution and increasing the selectivity of CO. [72] The results indicated that Zn‐doped Ga₂O₃ exhibited significant restraint on H₂ releasing from overall water splitting. Based on this result, they further deposited Ag onto the Zn–Ga₂O₃ to increase the production efficiency of CO because introduction of Ag as a cocatalyst could enhance the harvesting efficiency of sunlight and collect more photogenerated electrons for CO₂ reduction. In the case of Ag/Zn/Ga₂O₃, the selectivity toward CO evolution is higher than toward H₂ in the presence of NaHCO₃ solution, where the yields of CO and H₂ reached to 800 and 60 μmol g_cat⁻¹, respectively, over seven hours under UV irradiation. At the same time, the O₂ evolution was observed, which inferred that overall water splitting happened in this system while H₂ releasing was suppressed. These results indicate that the Ag‐modified Zn‐doped Ga₂O₃ realizes selective conversion of CO₂ and H₂O to CO and O₂ under UV irradiation.

Figure 2.13 Structures of CN before (a) and after (b) doping with carbon chains. (c) Electrochemical impedance spectroscopy (EIS) Nyquist plots under irradiation condition. Inset: Periodic on/off photocurrent response under visible‐light irradiation. (d) Comparison of the photocatalytic CO₂ reduction rate of the samples with different amount of glycine, respectively.

Source: Ren et al. [70].

On the other hand, it is reported that CO can originate from the secondary photolysis of unstable reduced products, such as HCO₂H. For example, Frei and coworker investigated the reaction mechanism of CO₂ photoreduction over Ti silicalite molecular sieve in the presence of methanol as electron donor by means of in situ FTIR [73]. It was found that HCO₂H, CO, and HCO₂CH₃ as reduced products were detected, in which mass proportion of CO was the highest. The formation of the products was studied through the infrared analysis of experiments with isotope‐labeled reactants, such as C¹⁸O₂, ¹³CO₂, and ¹³CH₃OH. The results inferred that the produced CO is derived from secondary photolysis of the reduced HCO₂H. In contrast, the formic acid is the primary two‐electron reduction product of CO₂ at the ligand‐to‐metal charge transfer transition (LMCT) excited Ti centers. This means that since the complex hydrocarbons are the target products, the photolysis effect should be paid more attention for suppressing the formation of CO.