Visible light-driven water oxidation by Ir oxide clusters coupled to single Cr centers in mesoporous silica

Visible light-induced water oxidation has been demonstrated at an Ir oxide nanocluster coupled to a single CrVI site on the pore surface of MCM-41 mesoporous silica. The photocatalytic unit was assembled by the reaction of surface Cr=O groups with Ir(acac)3 precursor followed by calcination at 300 degrees C and bond formation monitored by FT-Raman and FT-IR spectroscopy. High-resolution Z-contrast electron micrographs of the calcined material combined with energy-dispersive X-ray spot analysis confirmed the occlusion of Ir oxide nanoparticles inside the mesopores. Oxygen evolution of an aqueous suspension of the IrxOy-CrMCM-41 upon visible light irradiation of the CrVI-O ligand-to-metal charge-transfer absorption was monitored mass-spectrometrically. Comparison of the product yields for samples with low Cr content (Cr/Si 相似文献   

2D-C3N4 encapsulated perovskite nanocrystals for efficient photo-assisted thermocatalytic CO2 reduction

Very recently, halide perovskites, especially all-inorganic CsPbBr₃, have received ever-increasing attention in photocatalysis owing to their superior optoelectronic properties and thermal stability. However, there is a lack of study on their application in thermocatalysis and photo-thermocatalysis. Herein, we rationally designed a core–shell heterojunction formed by encapsulating CsPbBr₃ nanoparticles with the 2D C₃N₄ (m-CN) layer via a solid-state reaction (denoted as m-CN@CsPbBr₃). A series of experiments suggest that abundant adsorption and active sites of CO₂ molecules as well as polar surfaces were obtained by utilizing m-CN-coated CsPbBr₃, resulting in significant improvement in CO₂ capture and charge separation. It is found that the m-CN@CsPbBr₃ effectively drives the thermocatalytic reduction of CO₂ in H₂O vapor. By coupling light into the system, the activity for CO₂-to-CO reduction is further improved with a yield up to 42.8 μmol g⁻¹ h⁻¹ at 150 °C, which is 8.4 and 2.3 times those of pure photocatalysis (5.1 μmol g⁻¹ h⁻¹) and thermocatalysis (18.7 μmol g⁻¹ h⁻¹), respectively. This work expands the application of general halide perovskites and provides guidance for using perovskite-based catalysts for photo-assisted thermocatalytic CO₂ reduction.

A water-stable CsPbBr₃ catalyst is designed using core–shell encapsulation of the perovskite nanoparticle by 2D-C₃N₄ for photo-assisted thermocatalytic CO₂ reduction by H₂O. The m-CN@CsPbBr₃ heterojunction shows surprisingly high CO₂-to-CO yield.

The reduction of CO₂ into valuable hydrocarbon fuels via chemical catalytic processes to mitigate the greenhouse-effect has received wide attention.^1,2 Basically, the CO₂ molecule has a straight double-bonded non-polar structure, resulting in a huge activation energy barrier for the CO₂ reduction reaction (CRR).^3,4 Using a catalyst to adsorb and then activate the CO₂ molecules with free electrons is thus imperative.^5,6 Currently, there are three main types of catalytic CO₂ reduction reactions, i.e. thermocatalysis,⁷ electrocatalysis^8a,b and photocatalysis,^4b,c,5c where the catalysts used are referred to as thermocatalysts, electrocatalysts and photocatalysts, respectively. Traditionally, thermocatalysis is regarded as the conventional CRR process where the CO₂ molecules are well-activated and reduced by thermocatalysts using heat and the reductant agent H₂. Hence, thermocatalysis usually shows a relatively higher CRR efficiency compared to the other two cases.^7a,b,9a However, as it is very complex to safely transport and use H₂ for this process, it is more advantageous to use safe and free H₂O as the reductant if high thermocatalytic CRR efficiency can be achieved. Photocatalysis is regarded as an effective approach for CO₂ reduction owing to the merits of utilizing solar energy directly and low energy consumption. However, the efficiency of the photocatalytic CRR is still limited by the sluggish kinetics of CO₂ activation and H₂O dissolving.^4c,9b Very recently, Xu and co-workers reported a novel lead-free perovskite, Cs₃Sb₂I₉, for CO₂ reduction to CO and CH₄via photothermal synergistic catalysis without using any sacrificial agents or cocatalysts.^9c Unfortunately, the perovskite is not stable enough in the working solution. In addition, its activity is still limited. There is still some room to develop a simple method for synthesizing effective and stable photothermal catalysts.All-inorganic perovskites, for example cesium lead tribromide (CsPbBr₃), have emerged as a promising type of photocatalyst for the CRR, owing to their long photogenerated carrier diffusion length, tunable size, wide light-absorption range, etc.^5c,10 CsPbBr₃ perovskites have been widely explored in composites with other materials, such as g-C₃N₄,¹¹ graphene oxide^5a,12 and TiO₂ (ref. 13) to construct heterojunctions for photocatalytic CO₂ reduction. For example, Xu and his co-workers mixed CsPbBr₃ with NH_x-rich g-C₃N₄ nanosheets to construct a heterojunction with a fast carrier-transfer bridge for improved charge separation and hence enhanced photocatalytic activity for CO₂ reduction.^14a Similarly, Lu''s group reported the coating of CsPbBr₃ with graphdiyne as a physical protection layer to tackle the stability issue in photocatalytic CO₂ reduction in H₂O vapor, given that the perovskite shows a low tolerance towards water.^14b It should be mentioned that most of the reported halide perovskite nanocrystals (NCs) were prepared by the solution method using organic solvents, which is troublesome for large-scale production.^15a,b Besides, considering that thermocatalysis usually outperforms photocatalysis in CO₂ reduction, it would be intriguing to explore the catalytic activity of CsPbBr₃ under coupled thermal and irradiation effects. To the best of our knowledge, there are few reports of water-stable CsPbBr₃-based thermal CRR with H₂O as a reducing agent.Herein, using a molten-salt method, we elaborately encapsulate CsPbBr₃ with a 2D m-CN layer to construct a water-stable heterojunction for photo-assisted thermal catalytic CO₂ reduction. We found that under pure thermocatalysis conditions, m-CN@CsPbBr₃ showed the ability to drive the CRR reduction to CO using the CO₂ and H₂O as reactants. Moreover, with the introduction of simulated solar illumination, the corresponding reduction yield rises to 42.8 μmol g⁻¹ h⁻¹, which is 2.3 and 8.4 times those of pure thermocatalysts and photocatalysts, respectively. This is the first report on the fabrication of CsPbBr₃ perovskite NCs by means of a solid-phase reaction. This work expands the application of CsPbBr₃ perovskite and can help us better understand CO₂ reduction by H₂O.A molten-salt method, where ion salts act as a high-temperature liquid solvent to accelerate the dissolution of raw materials, the transport of reactants, and the directional assembly of basic units, was used to prepare perovskite catalysts with a specific morphology.^15c It is worth noting that melem can be obtained by thermal condensation of urea below 450 °C, while g-C₃N₄ can be acquired in the temperature range between 480 °C and 550 °C (Fig. S1a^†).^16a Therefore, the composition of the coating layer may be assigned to an intermediate product (named m-CN) possessing many edge NH_x groups between melem and g-C₃N₄, which is attributed to the higher local temperature (>450 °C) caused by the molten salt. To confirm this, a comparison study is performed at 420 °C and 470 °C for the synthesis of m-CN@CsPbBr₃ catalysts. As revealed in Fig. S1b,^† the best preparation temperature is 450 °C for m-CN@CsPbBr₃ catalysts, while the phase transition of CsPbBr₃ occurs at 470 °C. During the heat treatment, CsPbBr₃ was changed into the solution state, i.e. molten salt (Fig. S2^†). Meanwhile the polymerized monomers of carbon nitride adsorbed onto the surface of CsPbBr₃, and then the micron-sized CsPbBr₃ was “cut” into nanoparticles by encapsulation with m-CN coatings, as shown in Fig. 1a. To elucidate the formation process of m-CN@CsPbBr₃, different coatings with CsPbBr₃-to-urea mass ratios of 1 : 3, 1 : 5 and 1 : 10 were synthesized and are denoted as m-CN@CsPbBr₃-3, m-CN@CsPbBr₃-5 and m-CN@CsPbBr₃-10, respectively. The corresponding XRD and FTIR results also suggest the encapsulation of CsPbBr₃ with m-CN in these samples, as shown in Fig. S3a and b.^† It should be noted that the m-CN@CsPbBr₃-3 sample gives the typical peaks of g-C₃N₄ at 2-theta of 13 and 27°, while the broad XRD peaks of m-CN@CsPbBr₃-10 could be ascribed to melem, suggesting that a certain content of molten salt is required to improve the temperature profile for the thermal polymerization of the CN coating layers.^16b Fourier transform infrared (FTIR) spectroscopy of m-CN@CsPbBr₃-3 and m-CN@CsPbBr₃-10 showed similar peaks, where the peaks in the wide range from 1200 to 1700 cm⁻¹ are attributed to the skeleton signal of g-C₃N₄.^16c The absorption bands from 3000 to 3500 cm⁻¹ come from NH_x and –OH groups.^14a,16b In comparison to m-CN@CsPbBr₃-10, the normalized intensity of the band from 3000 to 3500 cm⁻¹ for m-CN@CsPbBr₃-3 is evidently decreased, which can be ascribed to the stronger interactions of edge NH_x groups and bromide anions of CsPbBr₃.¹⁶ It is also observed that the m-CN@CsPbBr₃-3 peaks shifted to lower values compared with m-CN@CsPbBr₃-10, as shown in the magnified FTIR spectra of Fig. S3,^† suggesting stronger interactions occurred with the introduction of more bromide anions.^14a,16d When the relative mass ratio of urea to CsPbBr₃ is changed, either the amount of loaded CsPbBr₃ particles is lower and sporadic (Fig. S4a,^† m-CN@CsPbBr₃-10) or the CsPbBr₃ particles are larger and exposed (Fig. S4b,^† m-CN@CsPbBr₃-3), whereas the m-CN@CsPbBr₃-5 sample displays completely uniform and coated CsPbBr₃ particles, suggesting that the appropriate ratio of urea to CsPbBr₃ is critical for optimal m-CN@CsPbBr₃ formation. Therefore, the m-CN@CsPbBr₃-5 catalyst with an optimized coating ratio was chosen for further study because it may be endowed with a more heterogeneous interface and excellent stability. For the synthesis of pure CsPbBr₃ for further composition, vacuum was employed to obtain less-defective CsPbBr₃. We also attempted to treat the mixture of CsPbBr₃ and urea in air. However, very little polymerization of urea to carbon nitride occurred, and thus, the N₂ condition was chosen for m-CN@CsPbBr₃ preparation. Therefore, we successfully encapsulated the perovskite CsPbBr₃ with a 2D m-CN layer by utilizing CsPbBr₃ as a molten salt under N₂ conditions. It is greatly emphasized that this is the first report on the fabrication of CsPbBr₃ perovskite NCs by means of a solid-phase reaction. Differing from the traditional synthesis approaches such as ball milling and solution processing strategies, this unique method can avoid some difficulties, including a complicated procedure and the formation of organic branches and undesired phases, showing the attractive application of this method for fabricating catalysts beyond perovskites.Open in a separate windowFig. 1(a) Illustration of the synthesis of m-CN@CsPbBr₃; (b) XRD patterns of m-CN@CsPbBr₃, the reference m-CN and CsPbBr₃; (c and d) TEM image and the corresponding HRTEM result of m-CN@CsPbBr₃, where the yellow arrows mark the thin m-CN layers.As a representative material, m-CN@CsPbBr₃-5 is selected to investigate the influence of encapsulating perovskite nanocrystals with 2D m-CN for boosting photo-assisted thermocatalytic CO₂ reduction. The XRD patterns in Fig. 1b show that the intensity of the peaks ascribed to CsPbBr₃ was obviously decreased after the m-CN encapsulation, suggesting that the crystallinity of CsPbBr₃ is reduced. Magnification of the diffraction region from 21.2 to 22° suggests that the m-CN@CsPbBr₃ sample exhibits a larger full-width at half maximum (FWHM) than bare CsPbBr₃ (Fig. S5^†), further indicating that the particle size was decreased by the calcination treatment. It should also be noted that the as-prepared CsPbBr₃ formed in a vacuum also showed a crystal-lattice orientation, given that the texture coefficient of the [200] peak is much higher than the standard value of 1, while it is less than 1 for CsPbBr₃ prepared in N₂ and air (Fig. S6a and b^†). It is noted that the texture coefficient of the [200] peak is in agreement with CsPbBr₃ prepared in N₂ after m-CN coating, which further proved the melt-crystallization process of CsPbBr₃. The FTIR spectra of m-CN@CsPbBr₃ and melem showed similar peaks (Fig. S7^†), and the normalized intensity of the band from 3000 to 3500 cm⁻¹ of m-CN@CsPbBr₃ is obviously decreased compared to the reference m-CN. Furthermore, compared with the pure m-CN layer, the peaks for m-CN@CsPbBr₃ located at 1250 cm⁻¹, corresponding to the typical stretching mode of aromatic C–N and C N heterocycles in m-CN, are systematically shifted to lower values in the magnified FTIR spectra in Fig. S7,^† suggesting that the edge NH_x groups interact with the bromide anion of CsPbBr₃via ionic bonding.^16,17aFig. 1c displays the transmission electron microscopy (TEM) image of the m-CN@CsPbBr₃ sample. Clearly, it shows that the CsPbBr₃ nanoparticles were uniformly encapsulated by the m-CN layer. The corresponding high-resolution transmission electron microscopy (HRTEM) images further show clear lattice fringes with a measured interplanar spacing of 0.3 nm, as shown in Fig. 1d, which can be assigned to the (002) plane of cubic CsPbBr₃. Additionally, it shows that nano-sized (ca. 8 nm) CsPbBr₃ particles were encapsulated by the m-CN layer in the m-CN@CsPbBr₃ catalyst (Fig. S8^†). In contrast, the reference CsPbBr₃, which was prepared without urea, is micron-sized (Fig. S9^†), further confirming that the particle size of CsPbBr₃ was changed by the in situ encapsulation with the m-CN layer. Fig. S10^† displays the scanning transmission electron microscopy (STEM) image with the corresponding elemental mapping results, which further confirm that the CsPbBr₃ nanoparticles are wrapped by the m-CN layer. The TEM images and the corresponding elemental mapping of the reference synthesized melem are shown in Fig. S11 and S12,^† respectively, and they resemble those of m-CN@CsPbBr₃, further confirming that the coating layer can be attributed to m-CN.X-ray photoelectron spectroscopy (XPS) was carried out to study the chemical environment of the elements in the m-CN@CsPbBr₃ heterojunction. Fig. S13a^† shows the C 1s XPS results of m-CN@CsPbBr₃ and reference m-CN samples. Two main peaks located at ca. 284.8 and 288.2 eV can be detected, and the first is assigned to the C–C in graphitic carbon, while the second comes from the N–C N coordination of triazine rings.^16,17b It should be noted that there is a slight shift of the N–C N signal of m-CN@CsPbBr₃ to higher binding energy as compared with the bare m-CN, indicating there is an interaction between CsPbBr₃ and m-CN. For the corresponding N 1s XPS curves, three main peaks centered at 398.7, 399.7 and 400.9 eV can be detected (Fig. S13b^†), and they can be assigned to C–N C, N–(C)₃ and C–N–H of the carbon nitride skeleton, respectively.¹⁷ However, the weak peak at ca. 404.4 eV is usually regarded as coming from positive charge localization in heterocycles.^17c It almost disappears in m-CN@CsPbBr₃ relative to bare m-CN, further suggesting that there is some chemical interaction between m-CN and CsPbBr₃ for neutralizing the positive charge.^14a Fig. S14a^† gives the Cs 3d XPS results of m-CN@CsPbBr₃ and bare CsPbBr₃. Two main peaks at 724.5 and 738.4 eV can be detected and come from the Cs 3d_5/2 and 3d_3/2 signals.^14,15 However, a slight shift to higher binding energy can be detected for m-CN@CsPbBr₃, further confirming the above mutual interaction between m-CN and CsPbBr₃. In the Pb 4f XPS result in Fig. S14b,^† there are two peaks at ca. 138.4 and 143.3 eV. Similarly, there is a shift to higher binding energy for m-CN@CsPbBr₃ compared with CsPbBr₃. Fig. S14c^† shows the Br 3d XPS results. Clearly, there are two fitted peaks located at ca. 68.3 and 69.3 eV. The sample of m-CN@CsPbBr₃ exhibits a ca. 0.2 eV shift to higher binding energy compared with bare CsPbBr₃. Both the shifts of Pb 4f and Br 3d in the XPS results suggest that chemical bonds of N–Br have been formed during the preparation process.^11b,14aThe UV-vis absorption spectra of the pristine m-CN and micro-sized CsPbBr₃ in Fig. 2a exhibit the typical absorption edges of CsPbBr₃ and m-CN at ca. 553 and 430 nm, corresponding to bandgaps of 2.24 and 2.88 eV, respectively. For the sample of m-CN@CsPbBr₃, two obvious edges were observed, which correspond to the light absorption edges of CsPbBr₃ and m-CN, indicating the successful combination of the two materials. Fig. S15^† displays the photoluminescence (PL) curves from 375 nm excitation. Strong emission at ca. 450 nm was marked for m-CN, and the peak at 524 nm is attributed to CsPbBr₃. There are two peaks in the PL curve of m-CN@CsPbBr₃, confirming the composition of the catalyst. Moreover, compared with the bare m-CN and CsPbBr₃, the PL intensity of m-CN@CsPbBr₃ is lower, which indicates suppressed carrier recombination in m-CN@CsPbBr₃. The time-resolved PL spectrum was further obtained to explore the carrier dynamics of m-CN@CsPbBr₃. Fig. S16a and b^† exhibit the transient PL spectra at 440 and 523 nm, where the signals can be considered as coming from m-CN and CsPbBr₃ (Fig. S15^†), respectively, and the corresponding fitted results are shown in Tables S1 and S2.^† Obviously, compared with pure m-CN at 440 nm (Fig. S16a^†), m-CN@CsPbBr₃ shows much faster decay after the introduction of CsPbBr₃. Interestingly, with respect to pure CsPbBr₃ at 523 nm (Fig. S16b^†), the opposite trend could be observed. The results testify that the photogenerated electrons of m-CN and holes of CsPbBr₃ will migrate towards each other under the effect of a built-in electric field, and the photogenerated electrons may be transported from CsPbBr₃ to the m-CN layer. The steady-state photovoltages (SPV) of the different samples were further compared to evidence the promoted charge separation in the m-CN@CsPbBr₃ heterojunction, as shown in Fig. 2b. Clearly, the samples of m-CN and CsPbBr₃ exhibited negligible SPV signals. Meanwhile, the m-CN@CsPbBr₃-5 sample demonstrated the highest positive photovoltage response compared with the other catalysts with coating layers (m-CN@CsPbBr₃-10 and m-CN@CsPbBr₃-3), suggesting that its surface is populated by a concentration of holes due to the high charge-separation efficiency (Fig. S17a^†).^18a Transient photovoltage (TPV) analysis was further carried out to study the migration dynamics of the photogenerated carriers.¹⁸ Two obvious positive response peaks (peaks 1 and 2) can be detected from the sample of m-CN@CsPbBr₃ under 400 nm illumination (Fig. S17b^†). Peak 1 can be assigned to a fast process related to the migration of photogenerated carriers in the built-in electric fields inside particles,^18b and thus the stronger intensity of peak 1 of m-CN@CsPbBr₃ suggests that more photogenerated carriers are transported to the interface between m-CN and CsPbBr₃. Peak 2 is thought to arise from carrier transport between particles.¹⁸ A further step to study the carrier transfer dynamics was to perform picosecond transient absorption spectroscopy (TA). Both the samples of bare CsPbBr₃ (Fig. S18a^†) and m-CN@CsPbBr₃ (Fig. S18b^†) exhibit obvious negative ground-state bleaching. Fig. 2c shows the corresponding TA kinetic plots monitored at 520 nm with a distinct difference in the delays. As listed in Table S3,^† the fitted average lifetime of m-CN@CsPbBr₃ (197.7 ps) is much longer than that of bare CsPbBr₃ (77.2 ps), consistent with the above transient PL and TPV results. It is concluded that the internal electric field between m-CN and CsPbBr₃ would promote the carrier transfer and separation in the m-CN@CsPbBr₃ heterojunction.¹⁹ According to the direction of carrier transfer and the strong built-in electric field, it can be inferred that an S-scheme heterojunction is formed between m-CN and CsPbBr₃.Open in a separate windowFig. 2(a) UV-Vis absorption spectra of m-CN@CsPbBr₃, the reference m-CN and CsPbBr₃; (b) surface photovoltage plots of the m-CN, CsPbBr₃ and m-CN@CsPbBr₃ samples; (c) transient absorption spectroscopy curves and lifetime fittings at 520 nm; (d) band structure of the m-CN@CsPbBr₃ heterojunction and the corresponding paths of CO₂ photo-reduction.In order to acquire stronger confirmation of the direction of carrier transfer, ultraviolet photoelectron spectroscopy (UPS) was performed to check the valence band (VB) potential and conduction band (CB) potential of the m-CN and CsPbBr₃ samples. The results and corresponding valence/conduction band potentials (vs. vacuum level) are given in Fig. S19^† and Table S4.^† The relative valence/conduction band positions of m-CN and CsPbBr₃ are shown in Fig. 2d, and they can be attributed to an S-scheme heterojunction, agreeing well with the aforementioned analysis. There is an offset of the bands of the m-CN and CsPbBr₃, and thus close contact between these two materials would cause band bending at the interface. Then, a built-in electric field will be formed making the m-CN and CsPbBr₃ centers of positive and negative charges, respectively. Moreover, the unique conduction/valence band structure of this heterojunction results in polar surfaces of m-CN (holes) and CsPbBr₃ (electrons). This built-in electric field would also facilitate the separation of photogenerated charges and thus suppress charge recombination.Considering that the m-CN@CsPbBr₃ catalyst possesses excellent photogenerated carrier separation ability, its photo-assisted thermocatalytic CO₂ reduction activity was then investigated. We evaluated the catalytic CO₂ reduction of the samples in a flow reactor with CO₂ and H₂O vapor as the reactants, such that the H₂O would continuously pass through the catalyst bed. During the flow reaction process, light and heat were applied to the reactor. Thermogravimetric analysis (TGA) was performed to check the temperature tolerance of the m-CN@CsPbBr₃ catalyst in nitrogen, as shown in Fig. S20.^† It indicates that the thermocatalytic CRR can take place at less than 200 °C because of the absence of any decomposition below this temperature. The instability of CsPbBr₃ to humidity has always been a huge obstacle for applications. The m-CN@CsPbBr₃ catalyst with the protection of the m-CN layer was then treated in pure water to check its chemical stability. As shown in Fig. S21a and b,^† after soaking in water for ∼0.5 and 17 h, there is almost no change in color; however, the bare CsPbBr₃ turns white after being in water for less than 1 h. The corresponding XRD results show no change from before to after the water immersion for ∼0.5 and 17 h. Meanwhile, bare CsPbBr₃ exhibited an obvious phase transition. Additionally, the water-treated samples were checked by TEM. As shown in Fig. S22,^† there is almost no change in morphology after the treatment of ∼0.5 and 17 h, in accordance with the above XRD results. Fig. S23^† shows the XPS results of the water-treated samples, and there are almost no changes for all the studied elements. These results suggest that m-CN@CsPbBr₃ exhibited superior water stability due to the m-CN encapsulation. Fig. 3a compares the CO₂ reduction activities of pristine m-CN, micro-sized CsPbBr₃ and m-CN@CsPbBr₃ under three different conditions, with our m-CN@CsPbBr₃ catalyst benefiting from superior durability. Under the pure simulated sunlight condition (Fig. 3a), the sample of m-CN@CsPbBr₃ displays the highest CO₂-to-CO yield of 8.15 μmol g⁻¹ h⁻¹, followed by CsPbBr₃ (4.5 μmol g⁻¹ h⁻¹) and m-CN (2.2 μmol g⁻¹ h⁻¹). For pure thermocatalysis at the temperature of 150 °C, the CO generation of m-CN@CsPbBr₃ is 22.4 μmol g⁻¹ h⁻¹, and those of the bare CsPbBr₃ and m-CN are 8.8 and 0 μmol g⁻¹ h⁻¹, respectively. More impressively, when the light and heat were coupled to drive photo-assisted thermocatalysis, the CO₂-to-CO yield of m-CN@CsPbBr₃ was increased to 42.8 μmol g⁻¹ h⁻¹, which is much higher than those of bare m-CN (5.1 μmol g⁻¹ h⁻¹) and CsPbBr₃ (18.7 μmol g⁻¹ h⁻¹). It is suggested that the light excitation would generate electrons and holes and these electrons and holes would migrate to the surfaces of CsPbBr₃ and m-CN, facilitating the thermocatalytic CRR process. Under all three conditions, i.e., photocatalysis, thermocatalysis and photo-thermocatalysis, the sample of bare CsPbBr₃ gives the lowest activity, further suggesting the significance of the heterojunction for promoting the CRR performance. The photo-thermocatalytic performance of the sample of m-CN@CsPbBr₃ is also comparable to those of the other systems, but exceeds those of most reported CsPbBr₃-based photocatalysts (Table S5^†). Moreover, we checked the impact of coating content on the catalytic activity of m-CN@CsPbBr₃ at 150 °C and 3 suns. As depicted in Fig. S24,^† in comparison with m-CN@CsPbBr₃-3 and m-CN@CsPbBr₃-10, m-CN@CsPbBr₃-5 delivers the highest CO₂-to-CO yield, possibly due to the existence of the enriched heterogeneous interface and strong built-in electric field. Meanwhile, the catalytic stability of m-CN@CsPbBr₃-5 was also tested during the 6 h period of operation (Fig. S25^†), and the result reveals that its reduction ability at the end is comparable to its original performance. Further, the XPS spectra before and after testing m-CN@CsPbBr₃-5 manifest that all elements are almost unchanged (Fig. S26^†), once again verifying its excellent catalytic stability, which is promising for practical application.Open in a separate windowFig. 3(a) The CO₂ reduction to CO performance under the three conditions of photocatalysis, thermocatalysis and photo-thermocatalysis. For the latter two, the temperature is set to 150 °C; (b) mass spectrometry (MS) result of ¹³CO produced over m-CN@CsPbBr₃ from the ¹³CO₂ isotope experiment under thermocatalysis (m/z, mass/charge ratio); (c) CO₂-TPD results of the reference m-CN and the m-CN@CsPbBr₃ catalyst.In addition, a small amount of H₂ was also measured as the reductive by-product from m-CN@CsPbBr₃ samples at different temperatures (Fig. S27^†). These results indicate that the coupling of photo and thermal effects profoundly promotes CO₂ reduction. Fig. 3b shows the result of an isotopic ¹³CO₂ labeling experiment under the thermocatalytic conditions. A clear peak at m/z = 29 (¹³CO) is observed, confirming that the generated CO originates from the thermocatalytic reduction of ¹³CO₂ rather than from contaminants. The corresponding screen shots of the raw mass spectra data are displayed in Fig. S28.^†To understand the promoted performance of m-CN@CsPbBr₃, the adsorption abilities of CO₂ and CO on m-CN and m-CN@CsPbBr₃ were studied. Fig. 3c exhibits the CO₂ temperature-programmed desorption (TPD) curves of m-CN and m-CN@CsPbBr₃, which yield the adsorbed amounts of CO₂ according to the integrated areas of the curves. It is found that m-CN@CsPbBr₃ has a higher adsorbed amount and thus more sites for CO₂ adsorption and activation compared to m-CM, which could be attributed to the polar surfaces of m-CN and CsPbBr₃. Moreover, the desorption peak of the m-CN@CsPbBr₃ catalyst is situated at higher temperature, suggesting that the corresponding adsorption sites can facilitate the CO₂ reduction due to a stronger interaction with the CO₂ molecules. In the CO₂-TPD curve of CsPbBr₃ in Fig. S29,^† a higher desorption temperature of ∼320 °C is observed compared with that of m-CN, which can be responsible for the enhancement of desorption for the m-CN@CsPbBr₃ catalyst. CO-TPD was also performed, and the results are shown in Fig. S30.^† Clearly, both the m-CN and m-CN@CsPbBr₃ samples showed weak CO adsorption, which is thought to benefit CO evolution. Measurements were further performed at two other temperatures (100 and 200 °C) to determine the effect of heating on the CRR efficiency. As shown in Fig. S31^† (100 °C) and Fig. S32^† (200 °C), the yields of CO₂-to-CO were lower than at 150 °C, which may be ascribed to the difference of CO₂ and CO adsorption at different temperatures. The result suggests that there is an optimal reaction temperature for suitable balance of adsorption/desorption of CO₂ and CO to obtain the highest performance of CO₂ reduction. In situ FTIR spectroscopy was further carried out to investigate the possible reaction pathways of the CRR under photo-assisted thermocatalysis conditions. It is evident that increased IR peaks emerge with increasing irradiation time from 0 to 30 minutes (Fig. 4a and b), in which the peaks at 1457 cm⁻¹ and 1646 cm⁻¹ could be assigned to b-HCO₃⁻, while the peaks at 1248 and 1695 cm⁻¹ came from the vibration of the carboxylate (CO₂⁻), and the peak at 1337 cm⁻¹ can be attributed to the bidentate carbonate (b-CO₃²⁻). Also, the peaks located at 1379 and 1507 cm⁻¹ matched well with monodentate carbonate groups (m-CO₃²⁻).²⁰ Meanwhile, for the photo-assisted thermocatalysis in Fig. 4b, almost the same peaks were observed except with relatively stronger peak intensities, corresponding to the improved CRR efficiency under the coupled photo-thermal effect. Note that the peak at 1695 increased in intensity and the peaks at 1457 cm⁻¹ and 1646 cm⁻¹ disappeared compared with the spectrum under bare thermocatalytic conditions, indicating that b-HCO₃⁻ ions on the surface are transformed to surface CO₂⁻ species instead with the assistance of light.Open in a separate windowFig. 4 In situ FTIR spectra of m-CN@CsPbBr₃ under (a) bare thermocatalytic and (b) photo-thermal conditions at ten-minute increments from 0 to 30 min (from top to bottom). (c) Schematic diagram of the possible reaction pathways of the CRR for the sample of m-CN@CsPbBr₃ under photo-thermal conditions.In addition, the m-CO₃²⁻ signals at 1507 cm⁻¹ were more obvious compared to those under bare thermocatalytic conditions, suggesting that the adsorption of the reaction intermediates was adjusted by light irradiation. This can be attributed to the strong light absorption of CsPbBr₃ according to the FTIR curves of the bare CsPbBr₃ sample under the above two conditions (Fig. S33a and b^†). This finding indicates the importance of encapsulating CsPbBr₃ for altering the formation of intermediates and improving CRR activity under photo-assisted thermocatalysis conditions. Moreover, the FTIR curves of the reference m-CN sample are similar to those of m-CN@CsPbBr₃ under bare thermocatalytic conditions (Fig. S34^†). Thus, the possible CRR pathways can be summarized as follows (Fig. 4c): (a) the H₂O molecules are first dissociated into H_ads and OH_ads groups when meeting hot carriers, i.e., holes generated from thermal or photo excitation; (b) the adsorbed CO₂ encounters OH_ads to generate HCO₃⁻ and m-CO₃²⁻ (step i); (c) then the conversion from surface m-CO₃²⁻ (step i) to CO₂⁻ species in the presence of H₂O is proposed, and the surface HCO₃⁻ (step i) species can be converted conveniently into CO₂⁻ under photo-assisted thermocatalysis conditions (step ii); for (c), the CO₂⁻ finally releases CO gas when it encounters H_ads and free electrons (step iii).Herein, a water-stable m-CN@CsPbBr₃ heterojunction was synthesized via a solid-state reaction, where bulk CsPbBr₃ micro-sized particles were melted and converted to nanoparticles with encapsulation by an m-CN coating during the calcination. The intimate contact between m-CN and CsPbBr₃ would induce band bending at the interface and form a built-in electric field, which would separate holes and electrons to m-CN and CsPbBr₃, respectively. As such a heterojunction with two charge poles, m-CN@CsPbBr₃ exhibited an excellent thermocatalytic CO₂-to-CO yield of 42.8 μmol g⁻¹ h⁻¹ under the assistance of irradiation, higher than that of pure photocatalysis (5.1 μmol g⁻¹ h⁻¹) or thermocatalysis (18.7 μmol g⁻¹ h⁻¹). This is the first report of photo-assisted thermocatalysis using CsPbBr₃-based materials. Our work thus expands the application of halide perovskites in CO₂ reduction.     

Visible light-driven electron transfer and hydrogen generation catalyzed by bioinspired [2Fe2S] complexes

Complexes [{(mu-SCH2)2NCH2C6H5}{Fe(CO)2L(1)}{Fe(CO)2L(2)}] (L(1) = CO, L(2) = P(Pyr) 3, 2; L(1) = L(2) = P(Pyr)3, 3) were prepared, which have the lowest reduction potentials for the mono- and double-CO-displaced diiron complexes reported so far. Hydrogen evolution, driven by visible light, was successfully observed for a three-component system, consisting of a ruthenium polypyridine complex, the biomimetic model complex 2 or 3, and ascorbic acid as both electron and proton donor in CH3CN/H2O. The electron transfer from photogenerated Ru(bpy)3(+) to 2 or 3 was detected by laser flash photolysis. Under optimal conditions, the total turnover number for hydrogen evolution was 4.3 based on 2 and 86 based on Ru(bpy)3(2+) in a three-hour photolysis.     

可见光促进二氧化碳参与的羧基化反应

二氧化碳是众所周知的温室气体, 也是重要的C1资源, 利用二氧化碳合成高附加值化合物具有重要意义. 其中, 羧酸类化合物广泛存在于天然产物、 药物、 日化品及工业原料中, 是一类非常重要的化合物. 因此, 利用二氧化碳合成羧酸类化合物是一个重要的研究方向; 另一方面, 由于二氧化碳反应活性低, 其转化通常需要高温等苛刻条件. 为解决该问题, 人们利用可见光作为能量来源, 可以在温和条件下实现二氧化碳的高效转化. 鉴于该方向近年来的蓬勃发展, 本文主要对可见光促进二氧化碳参与的羧基化反应进行介绍和总结, 按烯烃、 炔烃、 醛酮、 亚胺和(类)卤代物等重要的化工原料分类阐述, 并将各个反应的特点和机理将作为阐述的重点. 本文也对该领域的未来发展方向进行了展望, 希望为该领域的进一步发展提供参考.     

CdS/BiOBr Nanocomposite with Enhanced Activity under Visible Light for Photocatalytic Reduction of CO2 in Cyclohexanol

Kinetics and Catalysis - The CdS/BiOBr nanocomposites were synthesized through a hydrothermal deposition method. The structure and properties of the as-prepared nanocomposites were characterized by...     

Visible light induced CO2 reduction and Rh B decolorization over electrostatic-assembled AgBr/palygorskite

AgBr/palygorskite composite was prepared by an in situ electrostatic adsorption-deposition-precipitation method and characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), UV-Vis diffuse reflection, and BET surface measurements techniques. The layer negative charge and larger specific surface area of palygorskite, along with the poor cation-exchange ability of tetra-n-butyl ammonium cation (N(CH(2)CH(2)CH(2)CH(3))(4)(+)) due to its larger ion radius, could mainly account for high dispersity of AgBr on the surface of fibrous palygorskite. The rate of Rh B decolorization and CO(2) reduction with H(2) as a proton donor and reductant over AgBr/palygorskite was about three and two times faster than that of the corresponding bare AgBr, respectively. The strategy reported in this work can be easily extended to synthesize other palygorskite-based heterostructure catalysts.     

Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes

Electrocatalytic reduction of CO(2) by [Ru(tpy)(bpy)(solvent)](2+) (tpy = 2,2':6',2'-terpyridine, bpy = 2,2'-bipyridine) and its structural analogs is initiated by sequential 1e(-) reductions at the tpy and bpy ligands followed by rate limiting CO(2) addition to give a metallocarboxylate intermediate. It undergoes further reduction and loss of CO.     

Computational study of CO2 reduction by amines

Calculations at the MP2/aug-cc-pVDZ//MPWB1K/aug-cc-pVDZ level are reported for the reduction of CO2 by amines--primarily triethylamine. A polarizable continuum model is used to represent acetonitrile solvent for the reaction. Starting from a photochemically generated radical ion pair state, the mechanism of reduction is deduced to be one in which the CO2(.-) begins to use one of its oxygens to abstract a hydrogen from an alpha carbon of the amine radical cation. During this event, and before the transition state for H transfer is reached, the system encounters a surface crossing, which provides pathways for unproductive back electron transfer and for productive reduction, with the latter involving attachment of the hydrogen to the carbon of CO2. The result of the reduction is a closed-shell iminium formate ion pair, which completes the reaction by proton transfer between the ions, to give an eneamine and formic acid. On the basis of the calculations, approaches for improving the efficiency of the reduction and increasing the wavelength of the light used to drive the reaction are discussed. One of these modifications involves the use of a bicyclic amine as reductant.     

Alpha-cyclodextrin functionalized CdS nanocrystals for fabrication of 2/3 D assemblies

Different types of cyclodextrins (CDs) have been tested as mediators for the water phase transfer of organic-capped CdS nanocrystals (NCs), and alphaCD has been demonstrated to be the most effective system. The formation of a complex based on alphaCDs and colloidal NCs has been considered to be responsible for the phase transfer process and extensively investigated by optical, structural, and calorimetric measurements, as a function of the experimental parameters (pH and NC and CD concentration). A mechanism for the complexation phenomena has been suggested. The fabrication of 2/3 D supramolecular architectures has been proposed according to two different strategies. First, a layer-by-layer procedure has been used to obtain multilayered structures where polyelectrolyte layers have been intercalated with negatively charged alphaCD-CdS NC complexes by exploiting electrostatic interaction between polyelectrolyte and cyclodextrin OH groups. Second, a monolayer of CdS NCs has been deposited onto a self-assembled monolayer of sulfated CDs, thus combining the use of an electrostatic-force-based approach and host-guest chemistry. The important role played by host-guest interactions has then been revealed.     

利用光热耦合下的光电导研究TiO2光热催化还原CO2（英文）

利用太阳能缓解能源危机和解决环境污染,是当前和未来的全球性课题.其中,光催化技术的研究步伐日渐加快.这不仅体现在光催化材料种类的增加,更体现在以光催化为基础的多场协同催化,特别是光热耦合作用成为增强光催化性能的一种高效、可靠的方法.氧空位的引入不仅可以拓宽催化剂对可见光的吸收、抑制载流子的复合、促进反应物的吸附以及降低反应的活化能,而且对于光热协同催化效率的提升有着重要的贡献.然而,目前光热协同催化的表征多局限于常规的光催化手段.开展光热耦合下的测量技术对深刻理解光热催化是十分必要的.本文研究温度、气氛、氧空位浓度对TiO2光电导的影响,构建光电导与光热催化活性之间的关系.我们将商用的ST-01 TiO2制成浆料,利用丝网印刷法将浆料覆盖在刻有沟槽的FTO上,并通过N2/H2混合气不同温度退火,得到不同氧空位含量的TiO2薄膜(Ov-TiO2).采用紫外-可见光谱(UV-Vis),拉曼光谱(Raman),电子顺磁共振(ESR)等手段对样品进行了表征.结果表明,N2/H2退火温度越高,氧空位浓度越高.我们对不同浓度氧空位的样品进行了光催化及光热协同催化CO2还原实验.结果表明,适量氧空位的样品(H2-150)光催化还原CO2性能最差,但光热协同催化还原CO2的性能最佳.我们对其光电导值的衰减情况进行了分析,看到H2-150样品在CO2气氛、光热条件下,电导衰减加快.由于光电导的衰减是由电荷复合和电荷参与的表面反应共同决定的,为确定是哪一因素决定了电导的衰减,我们进一步测试了H2-150样品在N2气氛下的电导衰减情况.结果发现,H2-150样品在N2气氛、光热条件下电导衰减反而变慢.这表明,造成H2-150样品在CO2气氛、光热条件下的电导衰减加快是光热条件下CO2还原速率加快,也验证了H2-150具有较好的光热催化CO2活性.与H2-150样品不同的是,大量氧空位样品(H2-350)在CO2气氛、光热条件下电导衰减反而变慢,我们认为这是由于H2-350存在深能级缺陷,在热的作用下会将捕获的电子释放,因此延缓了光电导的衰减.但由于深能级电子的还原能力较弱,所以H2-350样品的光热CO2还原活性稍逊于H2-150.综上所述,在光热电导与光热催化相关的研究中,我们证实了在Ov-TiO2中被捕获的电子在热激发下可再次向导带弛豫,从而解释了Ov-TiO2优异的光热催化性能.因此,光热电导的研究在理解光热催化方面具有重要的前景.     

CdS:Mn nanocrystals passivated by ZnS: synthesis and luminescent properties

Synthesis and characterization of highly luminescent ZnS-passivated CdS:Mn (CdS:Mn/ZnS) core/shell structured nanocrystals are reported. Mn-doped CdS core nanocrystals are produced ranging from 1.5 to 2.3 nm in diameter with epitaxial ZnS shell of wider band gap via a reverse micelle process. UV irradiation-stimulated photo-oxidation of the ZnS shell results in formation of sulfate (ZnSO(4)) as determined by x-ray photoelectron spectroscopy, which increases the photoluminescence emission intensity and subsequent photostability. Luminescent relaxation lifetime data present two different decay components, consisting of slow decay emission from the Mn center and a fast decay emission from a defect-related center. The impact of the density of surface defect states upon the emission spectra is discussed.     

Visible light photocatalytic reduction of water using SrSnO3 sensitized by CuFeO2

SrSnO_3–δ, prepared in sealed ampoules, crystallizes in the perovskite structure. The band gap is directly allowed at 3.93 eV. The conductivity was found to change markedly and occurs by polaron hopping with activation energy of 0.22 eV. The thermal variation of the thermopower indicates an electron mobility μ_{e 300K} = 3.15∙10^–6 cm²∙V^–1∙s^–1), thermally activated. The capacitance measurement shows a linear behavior from which a flat band potential of –0.20 V_SCE and an electronic density of 5.56∙10¹⁸ cm^–3 were determined. The conduction band edge (–4.32 eV/–0.42 V_SCE) lies below the H₂O/H₂ level. Accordingly, SrSnO_3–δ can be used for water photoreduction when combined with the delafossite CuFeO₂ as sensitizer. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 3, pp. 160-166, May-June, 2009.     

CoII Cryptates Convert CO2 into CO and CH4 under Visible Light

Three Co^II octaazacryptates, with different substituents on the aromatic rings (Br, NO₂, CCH), were synthesised and characterised. These and the already published non-substituted cryptate catalysed CO₂ photoreduction to CO and CH₄ under blue visible light at room temperature. Although CO was observed after short irradiation times and a large range of catalyst concentrations, CH₄ was only observed after longer irradiation periods, such as 30 h, but with a small catalyst concentration (25 nm ). Experiments with ¹³C labelled CO₂ showed that CO is formed and reacts further when the reaction time is long. The CCH catalyst is deactivated faster than the others and the more efficient catalyst for CH₄ production is the one with Br. This reactivity trend was explained by an energy decomposition analysis based on DFT calculations.     

Enhanced solid-state electrochemiluminescence of CdS nanocrystals composited with carbon nanotubes in H2O2 solution

CdS nanoparticles composited with carbon nanotubes not only enhances their electrochemiluminescent intensity but also decreases their ECL starting potential; such a property would promote the application of quantum dots in fabricating sensors for chemical and biochemical analysis.     

Visible Light Induced Water Cleavage in CdS Dispersions Loaded with Pt and RuO2, Hole Scavenging by RuO2

Illumination of aqueous CdS dispersions loaded with Pt and RuO₂ by visible light produces hydrogen and oxygen in stoichiometric proportion. No degradation of the photocatalyst is noted after 60 h of irradiation time. The RuO₂ deposit on the particle surface greatly accelerates the transfer of holes from the semiconductor valence band to the aqueous solution thus inhibiting photocorrosion.     

Photosynthetic Fixation of CO2 in Alkenes by Heterogeneous Photoredox Catalysis with Visible Light

Light-driven fixation of CO₂ in organics has emerged as an appealing alternative for the synthesis of value-added fine chemicals. Challenges remain in the transformation of CO₂ as well as product selectivity due to its thermodynamic stability and kinetic inertness. Here we develop a boron carbonitride (BCN) with the abundant terminal B/N defects around the mesoporous walls, which essentially enhances surface active sites as well as charge transfer kinetics, boosting the overall rate of CO₂ adsorption and activation. In this protocol, anti-Markovnikov hydrocarboxylation of alkenes with CO₂ to an extended carbon chain is achieved with good functional group tolerance and specific regioselectivity under visible-light irradiation. The mechanistic studies demonstrate the formation of CO₂ radical anion intermediate on defective boron carbonitride, leading to the anti-Markovnikov carboxylation. Gram-scale reaction, late-stage carboxylation of natural products and synthesis of anti-diabetic GPR40 agonists reveal the utility of this method. This study sheds new insight on the design and application of metal-free semiconductors for the conversion of CO₂ in an atom-economic and sustainable manner.     

Visible light driven photocatalytic hydrogen evolution over CdS incorporated mesoporous anatase TiO2 beads

Research on Chemical Intermediates - The synthesis of CdS incorporated mesoporous anatase TiO2 beads is reported in this work. The mesoporous structure, crystalline structure, morphology and...     

钴联吡啶配合物在硅电极表面高效光电催化二氧化碳还原反应（英文）

光驱动二氧化碳还原实现可再生能源转化近年来引起普遍关注.利用小分子金属配合物电催化剂和吸光半导体材料构建的光电催化体系兼具电催化剂的高选择性和光电极的高光电转化效率等优点,在能源催化领域的应用日益广泛.已有将贵金属配合物催化剂用于光电催化二氧化碳还原的研究报道,但催化剂成本较高且制备方法不简便,在规模化实际应用中受到局限.基于早期的研究报道,我们发现非贵金属多联吡啶铁钴镍配合物在乙腈电解质中能高选择性电催化还原二氧化碳.结合半导体材料的特异性电荷分离性能从而将光能高效转化为电能驱动催化反应进行,我们选择廉价且易于制备的多联吡啶钴配合物催化剂,利用半导体硅晶片光电极,实现了均相体系二氧化碳的高效光电催化还原.我们采用电化学循环伏安法和恒电位电解法分别研究了催化剂在干燥和加水电解质环境中的催化还原行为,并且进一步研究了微量质子源的加入对半导体界面催化过程的影响,从而提出一种能改善半导体光电催化体系选择性的新方法.首先我们构建了电化学三电极体系,研究了在暗环境下三联吡啶钴和二联吡啶钴这两种配合物催化还原二氧化碳的电流密度和电解产物分布情况.由循环伏安曲线发现,这两种配合物都有两组催化还原峰,第二个基于吡啶配体还原的峰具有明显的催化特性.少量水的加入能进一步增加催化电流强度,而三联吡啶钴配合物的催化增强效果更加显著.在变扫速条件下将电流密度对扫速平方根进行归一化处理,发现无论在干燥环境还是少量加水环境下,两种催化剂的归一化电流密度均随扫速降低而明显增强,证明了催化剂具有电催化特性.推测水的催化增强作用可能与质子化电催化过程活性中间体有关.恒电位电解结果说明电催化产物以一氧化碳为主.基于上述研究,我们构建了光电化学三电极体系,以单晶硅片为工作电极,研究了在光照环境下这两种配合物催化还原二氧化碳的电流密度和电解产物分布情况.研究发现,催化剂对二氧化碳仍具有催化活性,光电压为400 m V.不同于硅线电极加水导致产氢,改用少量甲醇做质子源后,光电流强度进一步增强,竞争性产氢受到了抑制,从而使一氧化碳的法拉第效率得到显著提高,分别优化为94%和83%,并且光电流在14h内保持稳定.推测甲醇质子源的催化增强作用可能是与改变光电极液接界面传质动力学过程有关.