Academician Feng: Major Breakthrough in Photocatalyzing CO₂ to Reduce CO & CH₄
From：
Zhonglin International Group Date：07-17 953 Belong to：Industry Related
Recently, Academician Feng Xinliang of TU Dresden (corresponding author) and others reported a general method to promote the reduction of CO2 in ultra-thin copper (Cu) based hydrotalcite hydroxy salts driven by infrared (IR) light. Firstly, the author predicted the existence of split d-orbitals and their potential infrared response ability in two-dimensional (2D) hydrotalcite like hydroxy salts by simulating density of states (DOS), band structure, and optical absorption spectra. Then, the ultrathin basic Copper(II) sulfate (Cu4 (SO4) (OH) 6) nanosheets (CSOD) were taken as the prototype, and the infrared driven CO2 reduction reaction was taken as an example to verify the possibility of d-d transition induced photocatalytic reaction. Among them, the alkaline OH groups on the ultra-thin CSOD surface provide abundant sites for the initial adsorption of CO2.
X-ray absorption near edge structure (XANES) analysis and Eger electron spectroscopy analysis confirmed the existence of low coordination tetrahedron Cu in CSON, which can be used as the Active site for CO2 activation and continuous Protonation. The UV vis NIR spectrum and Gibbs free energy diagram indicate that the tetrahedral Cu site not only enhances infrared absorption, but also reduces the energy barrier of CO2 reduction reaction. By combining Tauc diagram, Density functional theory (DFT) calculation and in-situ FTIR spectra, the author proposed a parity forbidden Electron transfer photocatalysis mechanism of d-d orbital activation under infrared light irradiation.
The results showed that under infrared light irradiation, the generation rates of CO and CH4 in ultra-thin CSON were 21.95 and 4.11&mu, respectively; Mol g− 1 H− 1. It is superior to most catalysts under the same reaction conditions that have been reported so far (with a yield of less than 20 mol g− 1 h− 1). By preparing and uating various 2D hydrotalcite hydroxy salts, such as Cu2 (NO3) (OH) 3 nanosheets (CNON), Cu3 (PO4) (OH) 3 nanosheets (CPON) and Cu2 (CO3) (OH) 2 nanosheets (CCON), the author further proved the universality of infrared light driven CO2 reduction based on d-d Electron transfer mechanism, providing a powerful high-performance catalyst system for photocatalytic CO2 reduction.
Research background
Artificial photocatalysis provides new solutions to address the greenhouse effect and energy shortages, including the direct conversion of CO2 and water into fuel and oxygen under environmental conditions. Among them, infrared (IR) light has low energy and is prone to local heat generation, but its proportion in the solar spectrum is relatively high (about 50%), prompting people to explore ways to utilize it. The Electronic band structure engineering provides a feasible strategy to fine tune the energy band distribution and optimize the transfer behavior of activated electrons. Its gradual electronic transition mode can trigger the simultaneous conversion of CO2 and water into hydrocarbons and oxygen under infrared light irradiation.
Although ultra-thin metal catalysts are effective catalysts, metal conductors often undergo severe photo excited carrier recombination, resulting in low CO2 photoreduction efficiency. At the same time, the severe electron scattering effect and local plasma effect of metal conductors can cause energy loss of active electrons and local lattice heating, leading to catalyst deactivation. In addition, defect induced semiconductors with intermediate energy bands have certain infrared driven CO2 reduction performance, but limited material selection and harsh defect adjustment processes hinder their further development. Therefore, there is an urgent need to study a photocatalytic system that conforms to the general principle of streaming transitions under infrared light.
Figure 1. Possible electron transition mechanism of IR light driven CO2 reduction. (a) Metallic catalysts. (b) A semiconductor with an intermediate band. (c) Hydrotalcite like hydroxyl salts with d-d orbital transitions. The green sphere represents electrons, the white sphere represents holes, the red lightning represents IR light irradiation, the red arrow represents Electron transfer pathway, CB represents conduction band, and VB represents valence band.
There are various types of transition metal ion complexes that are easy to prepare. Due to d-d band transitions, they exhibit high infrared light absorption ability. However, in some transition metal complexes with tetrahedral or Octahedron coordination, such as hydrotalcite like hydroxy salts, the strong p-d orbital coupling between the ligand and the metal ions will lead to the transition metal valence band d orbital degeneracy. Research has found that hydrotalcite-like hydroxyl salts with two-dimensional (2D) nanosheet structures can serve as good candidates for infrared absorption and charge carrier separation, allowing activated electrons to participate in photocatalytic reactions.
Firstly, the author constructed a two-dimensional (2D) CSON model with single cell thickness for theoretical simulation. Selecting the [100] surface as the exposed surface, comparing the [010] surface with the [001] surface, the [100] surface is expected to have the highest surface energy, indicating its most excellent catalytic reaction activity.
In addition, the calculated optical absorption spectrum shows that there is an obvious absorption edge in the infrared region, which may be caused by d-d Electron transfer. The intrinsic bandgap of photon absorption is at 3. 0 eV, indicating that the oxidation Reduction potential of CSON is large enough to catalyze CO2 reduction and water oxidation at the same time.
In addition, there is a significant splitting d-orbitals in the downward DOS bandgap of the spin of CSON. According to the projected density of states (PDOS), the occurrence of empty d-bands in the bandgap is attributed to the degeneracy of d-orbitals in Cu atoms. Therefore, the d-d electron transition mechanism and appropriate bandwidth of 2D CSON provide high potential for infrared absorption and potential activity for infrared driven CO2 photoreduction.
Figure 3. Characterization of c-CSON. (a) TEM diagram. (b) HRTEM diagram, where the crystal plane spacing of 0.215 nm and 0.233 nm matches well with the spacing of d003 and d040, and corresponds to 90° The angle is very consistent with the calculated angle between the (003) and (040) crystal planes. (c) Circular dark field TEM image and corresponding element mapping imaging, with a scale of 200 nm. (d) AFM diagram, where the thickness of the hole rich surface nanosheets is 1.31 nm. (e) Cu K-edge XANES spectrum, with an enlarged image of the green circle in the illustration. (f) The Cu LMM Auger electron spectra of p-CSON and c-CSON show a significant increase in the proportion of Cu (I) components in the calcined samples.
All FTIR spectra were recorded on Thermo Scientific Nicolet iS50. The spectrum is displayed in the transmission unit, with a resolution of 4 cm&minus obtained through 64 scans; 1's spectrum. The dome of the reaction pool has two KBr windows that allow infrared transmission, while the third window allows radiation to be transmitted through liquid photoconductors connected to the same infrared lamp.
Firstly, add catalyst powder to the reaction pool, and then spray a small amount of water on the surface of the catalyst. After degassing in N2 atmosphere for 20 minutes, switch to high-purity CO2 flow, and seal the reaction tank after adsorption saturation. Finally, the FTIR spectrum was recorded as a function of time to study the adsorption of reactants in the dark and the desorption/conversion kinetics under irradiation.
Figure 4. Electronic band structure of ultra-thin 2D CSON and its IR light driven CO2 reduction performance. (a) UV vis NIR diffuse reflectance spectroscopy. (b) Tauc curves of p-CSON and (c) c-CSON. (d), e) The SRPES valence band and Secondary electrons cut-off spectrum of c-CSON, where VBM is at 3.09 eV, and the cut-off energy of Secondary electrons is 36.17 eV. (f) The schematic diagram of the electronic Electronic band structure of c-CSON, where the blue arrow indicates the electronic transition process. (g) The yield of photocatalytic CO2 reduction to CO and CH4 under different catalysts and conditions. (h) C-CSON in h&nu= SVUV-PIMS spectra of the products after 13CO2 photoreduction under 14.5 eV conditions. (i) The photocatalytic CO2 reduction to CO and CH4 cycle performance of c-CSON.
Mechanism study Based on the intermediates detected by in-situ FTIR, the author constructed theoretical models of different Reaction intermediate, calculated the corresponding Gibbs free energy, and determined the reaction path and energy barrier of c-CSON and p-CSON in the process of infrared light driven CO2 reduction. The author first calculated the CO2 adsorption energy at the OH and Cu sites in CSON. CO2 molecules tend to adsorb on surface OH sites rather than Cu sites, as the former has lower adsorption energy.
More importantly, compared to p-CSON, c-CSON with abundant surface vacancies has a specific surface area of about 2 times, which is beneficial for CO2 absorption. Subsequently, the adsorbed CO2 * will move to the nearby Cu site to be activated and Protonation, forming COOH * intermediates, which are considered as rate determining steps (RDS). In addition, the calculated Bader charge and charge density distribution show that the electron density of the Cu site on the c-CSON surface is higher than that of the original sample, which is conducive to CO2 activation and continuous Protonation process.
Figure 5. The mechanism of ultra-thin 2D CSON for IR light driven CO2 reduction. (a) In situ FTIR spectra of co adsorbed CO2 and H2O vapor mixtures on c-CSON under light irradiation. (b) Gibbs free energy diagram and (c) intermediate structure of c-CSON during CO2 reduction under IR light irradiation.
X-ray absorption near edge structure (XANES) analysis and Eger electron spectroscopy analysis confirmed the existence of low coordination tetrahedron Cu in CSON, which can be used as the Active site for CO2 activation and continuous Protonation. The UV vis NIR spectrum and Gibbs free energy diagram indicate that the tetrahedral Cu site not only enhances infrared absorption, but also reduces the energy barrier of CO2 reduction reaction. By combining Tauc diagram, Density functional theory (DFT) calculation and in-situ FTIR spectra, the author proposed a parity forbidden Electron transfer photocatalysis mechanism of d-d orbital activation under infrared light irradiation.
The results showed that under infrared light irradiation, the generation rates of CO and CH4 in ultra-thin CSON were 21.95 and 4.11&mu, respectively; Mol g− 1 H− 1. It is superior to most catalysts under the same reaction conditions that have been reported so far (with a yield of less than 20 mol g− 1 h− 1). By preparing and uating various 2D hydrotalcite hydroxy salts, such as Cu2 (NO3) (OH) 3 nanosheets (CNON), Cu3 (PO4) (OH) 3 nanosheets (CPON) and Cu2 (CO3) (OH) 2 nanosheets (CCON), the author further proved the universality of infrared light driven CO2 reduction based on d-d Electron transfer mechanism, providing a powerful high-performance catalyst system for photocatalytic CO2 reduction.
Research background
Artificial photocatalysis provides new solutions to address the greenhouse effect and energy shortages, including the direct conversion of CO2 and water into fuel and oxygen under environmental conditions. Among them, infrared (IR) light has low energy and is prone to local heat generation, but its proportion in the solar spectrum is relatively high (about 50%), prompting people to explore ways to utilize it. The Electronic band structure engineering provides a feasible strategy to fine tune the energy band distribution and optimize the transfer behavior of activated electrons. Its gradual electronic transition mode can trigger the simultaneous conversion of CO2 and water into hydrocarbons and oxygen under infrared light irradiation.
Although ultra-thin metal catalysts are effective catalysts, metal conductors often undergo severe photo excited carrier recombination, resulting in low CO2 photoreduction efficiency. At the same time, the severe electron scattering effect and local plasma effect of metal conductors can cause energy loss of active electrons and local lattice heating, leading to catalyst deactivation. In addition, defect induced semiconductors with intermediate energy bands have certain infrared driven CO2 reduction performance, but limited material selection and harsh defect adjustment processes hinder their further development. Therefore, there is an urgent need to study a photocatalytic system that conforms to the general principle of streaming transitions under infrared light.
Figure 1. Possible electron transition mechanism of IR light driven CO2 reduction. (a) Metallic catalysts. (b) A semiconductor with an intermediate band. (c) Hydrotalcite like hydroxyl salts with d-d orbital transitions. The green sphere represents electrons, the white sphere represents holes, the red lightning represents IR light irradiation, the red arrow represents Electron transfer pathway, CB represents conduction band, and VB represents valence band.
There are various types of transition metal ion complexes that are easy to prepare. Due to d-d band transitions, they exhibit high infrared light absorption ability. However, in some transition metal complexes with tetrahedral or Octahedron coordination, such as hydrotalcite like hydroxy salts, the strong p-d orbital coupling between the ligand and the metal ions will lead to the transition metal valence band d orbital degeneracy. Research has found that hydrotalcite-like hydroxyl salts with two-dimensional (2D) nanosheet structures can serve as good candidates for infrared absorption and charge carrier separation, allowing activated electrons to participate in photocatalytic reactions.
Firstly, the author constructed a two-dimensional (2D) CSON model with single cell thickness for theoretical simulation. Selecting the [100] surface as the exposed surface, comparing the [010] surface with the [001] surface, the [100] surface is expected to have the highest surface energy, indicating its most excellent catalytic reaction activity.
In addition, the calculated optical absorption spectrum shows that there is an obvious absorption edge in the infrared region, which may be caused by d-d Electron transfer. The intrinsic bandgap of photon absorption is at 3. 0 eV, indicating that the oxidation Reduction potential of CSON is large enough to catalyze CO2 reduction and water oxidation at the same time.
In addition, there is a significant splitting d-orbitals in the downward DOS bandgap of the spin of CSON. According to the projected density of states (PDOS), the occurrence of empty d-bands in the bandgap is attributed to the degeneracy of d-orbitals in Cu atoms. Therefore, the d-d electron transition mechanism and appropriate bandwidth of 2D CSON provide high potential for infrared absorption and potential activity for infrared driven CO2 photoreduction.
Figure 3. Characterization of c-CSON. (a) TEM diagram. (b) HRTEM diagram, where the crystal plane spacing of 0.215 nm and 0.233 nm matches well with the spacing of d003 and d040, and corresponds to 90° The angle is very consistent with the calculated angle between the (003) and (040) crystal planes. (c) Circular dark field TEM image and corresponding element mapping imaging, with a scale of 200 nm. (d) AFM diagram, where the thickness of the hole rich surface nanosheets is 1.31 nm. (e) Cu K-edge XANES spectrum, with an enlarged image of the green circle in the illustration. (f) The Cu LMM Auger electron spectra of p-CSON and c-CSON show a significant increase in the proportion of Cu (I) components in the calcined samples.
All FTIR spectra were recorded on Thermo Scientific Nicolet iS50. The spectrum is displayed in the transmission unit, with a resolution of 4 cm&minus obtained through 64 scans; 1's spectrum. The dome of the reaction pool has two KBr windows that allow infrared transmission, while the third window allows radiation to be transmitted through liquid photoconductors connected to the same infrared lamp.
Firstly, add catalyst powder to the reaction pool, and then spray a small amount of water on the surface of the catalyst. After degassing in N2 atmosphere for 20 minutes, switch to high-purity CO2 flow, and seal the reaction tank after adsorption saturation. Finally, the FTIR spectrum was recorded as a function of time to study the adsorption of reactants in the dark and the desorption/conversion kinetics under irradiation.
Figure 4. Electronic band structure of ultra-thin 2D CSON and its IR light driven CO2 reduction performance. (a) UV vis NIR diffuse reflectance spectroscopy. (b) Tauc curves of p-CSON and (c) c-CSON. (d), e) The SRPES valence band and Secondary electrons cut-off spectrum of c-CSON, where VBM is at 3.09 eV, and the cut-off energy of Secondary electrons is 36.17 eV. (f) The schematic diagram of the electronic Electronic band structure of c-CSON, where the blue arrow indicates the electronic transition process. (g) The yield of photocatalytic CO2 reduction to CO and CH4 under different catalysts and conditions. (h) C-CSON in h&nu= SVUV-PIMS spectra of the products after 13CO2 photoreduction under 14.5 eV conditions. (i) The photocatalytic CO2 reduction to CO and CH4 cycle performance of c-CSON.
Mechanism study Based on the intermediates detected by in-situ FTIR, the author constructed theoretical models of different Reaction intermediate, calculated the corresponding Gibbs free energy, and determined the reaction path and energy barrier of c-CSON and p-CSON in the process of infrared light driven CO2 reduction. The author first calculated the CO2 adsorption energy at the OH and Cu sites in CSON. CO2 molecules tend to adsorb on surface OH sites rather than Cu sites, as the former has lower adsorption energy.
More importantly, compared to p-CSON, c-CSON with abundant surface vacancies has a specific surface area of about 2 times, which is beneficial for CO2 absorption. Subsequently, the adsorbed CO2 * will move to the nearby Cu site to be activated and Protonation, forming COOH * intermediates, which are considered as rate determining steps (RDS). In addition, the calculated Bader charge and charge density distribution show that the electron density of the Cu site on the c-CSON surface is higher than that of the original sample, which is conducive to CO2 activation and continuous Protonation process.
Figure 5. The mechanism of ultra-thin 2D CSON for IR light driven CO2 reduction. (a) In situ FTIR spectra of co adsorbed CO2 and H2O vapor mixtures on c-CSON under light irradiation. (b) Gibbs free energy diagram and (c) intermediate structure of c-CSON during CO2 reduction under IR light irradiation.
