Preparation method and application of Z-type heterojunction composite catalyst

Abstract

The present invention belongs to the technical field of photoelectrocatalytic carbon dioxide reduction and discloses a preparation method of a Z-type heterojunction composite catalyst, comprising the following steps: dispersing potassium hexatantalate in an anhydrous ethanol solution of cobalt phthalocyanine, stirring, ultrasonicating, and evaporating the anhydrous ethanol; calcining the obtained solid product at 300°C for 2h to obtain K8Ta6O 19 / CoPc composite catalyst. The present invention uses a polyacid K8Ta6O rich in oxygen vacancies 19 By introducing cobalt phthalocyanine, the two semiconductors form a Z-type heterojunction, which can effectively reduce the electron-hole recombination rate, increase the carrier mobility rate, and also contain more oxygen vacancies and a larger electrochemically active surface area, which can provide more active sites and produce strong interactions with CO2 molecules, thereby greatly improving the photoelectrocatalytic carbon dioxide reduction performance of the catalyst.

Description

Technical Field

[0001] The present invention belongs to the technical field of photoelectrocatalytic carbon dioxide reduction and relates to a preparation method and application of a Z-type heterojunction composite catalyst. Background Art

[0002] Photoelectrocatalytic reduction of carbon dioxide (CO2) to valuable fuels or chemicals is considered one of the most promising approaches to achieving carbon neutrality and a solar-based economy. However, due to the inertness of CO2 and the complex electron-coupled proton transfer during the reduction process, the selectivity of catalysts for products and the CO2 conversion efficiency have fallen short of expectations.

[0003] Metallophthalocyanine (MPc, M such as Fe, Co, Ni, Mn, etc.) catalysts have excellent photoelectrocatalytic carbon dioxide reduction (CO2RR) performance. In particular, cobalt phthalocyanine (CoPc) has been shown to have high stability and catalytic activity in the photoelectrocatalytic CO2RR process due to its well-defined catalytic site (CoN4) and tunable structure. However, the electron transfer efficiency of CoPc during the catalytic process is generally low, and its CO2 adsorption and activation properties are insufficient, which greatly limits the catalytic efficiency of CoPc and cannot meet the needs of practical applications. Modification of the CoPc structure (such as adding conductive agents, hybridization with carbon materials, introduction of functional groups, or formation of heterojunction complexes) is an effective measure to improve its catalytic performance, but currently there is little research in this area. Since electronic tuning of the adsorption / activation interaction between CO2 and CoPc is an important prerequisite for improving the performance of photoelectrocatalytic CO2RR, if the adsorption / activation interaction between CO2 and CoPc can be enhanced, its photoelectrocatalytic CO2RR performance can be greatly improved. Summary of the Invention

[0004] The present invention aims to solve the technical problems of low electron transfer efficiency and insufficient carbon dioxide adsorption / activation performance in the process of cobalt phthalocyanine photoelectrocatalytic CO2RR, and provides a preparation method and application of a Z-type heterojunction composite catalyst. 19 By introducing cobalt phthalocyanine, the two semiconductors form a Z-type heterojunction, which can effectively reduce the electron-hole recombination rate, increase the carrier mobility rate, and also contain more oxygen vacancies and a larger electrochemically active surface area, which can provide more active sites and produce strong interactions with CO2 molecules, thereby greatly improving the photoelectrocatalytic carbon dioxide reduction performance of the catalyst.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In one aspect, the present invention provides a method for preparing a Z-type heterojunction composite catalyst, comprising the following steps: dispersing potassium hexatantalate in an anhydrous ethanol solution of cobalt phthalocyanine, stirring, ultrasonicating, evaporating the anhydrous ethanol, and then calcining the obtained solid product at 300°C for 2h to obtain K8Ta6O 19 / CoPc composite catalyst.

[0007] In the above technical solution, the ratio of cobalt phthalocyanine to potassium hexatantalate is 1:6-8.

[0008] In the above technical solution, the usage ratio of anhydrous ethanol to cobalt phthalocyanine is 40 mL:0.025 g.

[0009] In the above technical solution, the evaporation temperature of the anhydrous ethanol is 95°C.

[0010] On the other hand, the present invention also provides the use of the Z-type heterojunction composite catalyst prepared by the above preparation method in the photoelectrocatalytic CO2 reduction reaction.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] The present invention converts the polyacid K8Ta6O rich in oxygen vacancies into 19 By introducing cobalt phthalocyanine into the catalyst, the two semiconductors form a Z-type heterojunction, which can effectively reduce the electron-hole recombination rate, increase the carrier mobility, and also contain more oxygen vacancies and a larger electrochemically active surface area, which can provide more active sites and produce a strong interaction with CO2 molecules, thereby greatly improving the photoelectrocatalytic carbon dioxide reduction performance of the catalyst. In addition, the polyacid K8Ta6O 19 The introduction of can effectively inhibit the agglomeration of cobalt phthalocyanine during the catalytic process and improve the stability of the material.

[0013] The mechanism of photoelectrocatalytic carbon dioxide reduction obtained by the composite catalyst of the present invention is a Z-type heterojunction electron transfer mechanism, which can greatly improve the separation efficiency of photogenerated carriers and can highly selectively convert CO2 into CO, with a Faradaic efficiency of up to 99.5% and a TOF value of up to 4661.6h -1 (-1.0V vs. RHE), and has excellent stability and can last for more than 12 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 CoPc, K8Ta6O 19 , Fourier transform infrared spectra, XRD patterns and Raman spectra of CoN-7 and CoT-X.

[0015] Figure 2 K8Ta6O 19Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of CoPc and composite catalyst CoT-7.

[0016] Figure 3 K8Ta6O 19 , CoPc and composite catalyst CoT-7.

[0017] Figure 4 K8Ta6O 19 and electron paramagnetic resonance spectra of CoT-7.

[0018] Figure 5 CoPc, K8Ta6O 19 And UV-visible absorption spectra and fluorescence spectra of the composite catalyst CoT-X.

[0019] Figure 6 For different K8Ta6O 19 Electrochemical characterization results of the added amount of composite catalyst CoT-X.

[0020] Figure 7 For different K8Ta6O 19 Photoelectrocatalytic CO2 reduction test results of the added amount of composite catalyst.

[0021] Figure 8 These are the stability test results of the composite catalyst CoT-7.

[0022] Figure 9 CoPc, K8Ta6O 19 Band gap width and Mott-Schottky diagram.

[0023] Figure 10 This is the photoelectrocatalytic mechanism of the composite catalyst of the present invention. DETAILED DESCRIPTION

[0024] The following examples are used to illustrate the present invention but are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. The test methods in the following examples are all conventional methods unless otherwise specified.

[0025] Lindqvist type K8Ta6O used in the embodiment of the present invention 19 See references (Filowitz M, Ho RKC, Klemperer WG, Shum W. 17O nuclear magnetic resonance spectroscopy ofpolyoxometalates.1.Sensitivity and resolution.Inorg Chem 1979;18:93-103) synthesis.

[0026] Example 1 Preparation of different K8Ta6O 19 Added amount of composite catalyst

[0027] Take 0.015g, 0.175g, and 0.2g K8Ta6O respectively 19 , dispersed in 40 mL of anhydrous ethanol solution containing 0.025 g of commercial cobalt phthalocyanine, stirred for 2 h, then ultrasonicated for 2 h, and evaporated the anhydrous ethanol at 95 ° C to obtain a blue solid. The obtained blue solid was placed in a muffle furnace and calcined at 300 ° C for 2 h to obtain cobalt phthalocyanine modified K8Ta6O 19 The composite catalyst CoT-X (X is K8Ta6O 19 addition ratio), marked as CoT-6, CoT-7, and CoT-8, respectively.

[0028] Comparative Example 1

[0029] Take 0.175g K8Ta6O 19 , dispersed in an anhydrous ethanol solution containing 0.025 g of commercial cobalt phthalocyanine, stirred for 2 hours, then sonicated for 2 hours. The anhydrous ethanol was evaporated at 95°C to obtain a blue solid. The resulting blue solid was calcined at 250°C for 2 hours to obtain a composite. The composite appeared as a blue powder solid and had poor performance upon testing.

[0030] Comparative Example 2

[0031] Take 0.175g K8Ta6O 19 , dispersed in an anhydrous ethanol solution containing 0.025g of commercial cobalt phthalocyanine, stirred for 2 hours, then sonicated for 2 hours. The anhydrous ethanol was evaporated at 95°C to yield a blue solid. The resulting blue solid was calcined at 350°C for 2 hours to obtain a composite. The color of the composite changed significantly to earthy gray, indicating that the structure of the composite catalyst had been destroyed. Performance tests showed that only hydrogen, not CO, was produced.

[0032] Comparative Example 3

[0033] Take 0.175g K7HNb6O 19 , dispersed in an anhydrous ethanol solution containing 0.025g of commercial cobalt phthalocyanine, stirred for 2h, then ultrasonicated for 2h, and evaporated anhydrous ethanol at 95℃ to obtain a blue solid. The obtained blue solid was calcined at 300℃ for 2h to obtain cobalt phthalocyanine modified K7HNb6O19 The composite catalyst is marked as CoN-7.

[0034] (1) Catalyst characterization:

[0035] The crystal structures of all catalysts were characterized by X-ray diffraction (XRD, Bruker D8, Germany) using Cu Kα radiation (λ = 0.15406 nm). The morphology of the samples was observed using field emission scanning electron microscopy (FESEM, JSM-7610F, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-F200, Japan). The surface electronic states of the catalysts were examined using X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, USA). The UV-visible diffuse reflectance spectra of the catalysts were measured at room temperature using BaSO4 as a reference using an Agilent Cary 5000, USA. The elemental composition of the samples was quantitatively analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5110, USA). Photoluminescence (PL) spectroscopy was measured using a spectrophotometer (F7000, Japan) at an excitation wavelength of 430 nm. Electron paramagnetic resonance (EPR) signals of oxygen vacancies were recorded at 77 K using a Bruker 300 EPR spectrometer. Fourier transform infrared (FT-IR) spectroscopy of the catalysts (Spectrum 2, USA) was measured using KBr pellets. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy of the catalysts was performed using a Thermo 8700 infrared spectrometer from Thermo Scientific.

[0036] Figure 1 CoPc, K8Ta6O 19 and Fourier transform infrared spectra, XRD spectra and Raman spectra of composite catalyst CoT-X. Figure 1 -a is CoPc, K8Ta6O 19 and infrared spectra of CoT-7; Figure 1 -b is CoPc, K8Ta6O 19 and XRD patterns of CoT-7; Figure 1 -c is K8Ta6O 19 and XRD patterns of CoT-X; Figure 1 -d is CoPc, K8Ta6O 19 and Raman spectra of CoT-7. Figure 1 -a, 846.5 and 677.7cm -1 The peak at is derived from K8Ta6O 19 Stretching vibration of the terminal Ta-O bond, 535.4 cm -1The peak at 728.4 cm is attributed to the stretching vibration of Ta-O-Ta. -1 It is the characteristic skeleton vibration absorption of the phthalocyanine ring, 1085.4cm -1 It is the vibration absorption of CH on the phthalocyanine ring, 914.5cm -1 It is the vibration absorption of Co-N bond, 1422.2 and 1475.6 cm -1 are the vibration absorption of C-C and C-N bonds, respectively, while 1525 and 1607.6 cm -1 It is the vibration absorption of C=N and pyrrole ring. In the infrared spectrum of CoT-7, no obvious characteristic peak of CoPc was found, which may be due to the small amount of CoPc added. Figure 1 In the XRD spectra of -b and 1-c, the original K8Ta6O 19 There are three obvious characteristic peaks at 9.9°, 10.8° and 29.9°. The characteristic peaks of CoPc mainly appear at 6.9° and 9.0°. For the composite CoT-6, CoT-7 and CoT-8, obvious K8Ta6O 19 The characteristic peak of CoPc can be observed at 6.9°. Figure 1 -d, 520 and 869cm -1 Belongs to K8Ta6O 19 The characteristic absorption peaks of CoPc can be clearly observed in the composite catalyst CoT-7 by comparing the Raman spectrum with that of CoPc, but no obvious characteristic peaks of polyacids can be found. Combining the above infrared and XRD, it can be concluded that cobalt phthalocyanine modified K8Ta6O 19 The composite catalyst has been successfully prepared.

[0037] Figure 2 K8Ta6O 19 Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of CoPc and composite catalyst CoT-7. Figure 2 -a~2-d are CoPc, K8Ta6O 19 , FESEM image of CoN-7 and FESEM magnified image of CoN-7, Figure 2 -e is the element mapping diagram of CoN-7. As can be seen from the figure, CoPc exhibits an irregular rod-like morphology ( Figure 2 -a), while K8Ta6O 19 It has a marble-like appearance ( Figure 2 -b). The surface of the composite catalyst CoT-7 is rough, but irregular rod-shaped CoPc can be observed attached to its surface ( Figure 2-c and 2-d), TEM-EDS elemental maps (i.e., mapping maps) show that C, N, Co, K, and Ta are uniformly distributed on CoT-7 ( Figure 2 -e), indicating that CoPc molecules are located in K8Ta6O 19 There is good dispersion on the surface, which also indicates the construction of heterojunction.

[0038] Figure 3 K8Ta6O 19 , CoPc and composite catalyst CoT-7. Figure 3 -a are K8Ta6O 19 , full XPS spectra of CoPc and CoT-7; Figure 3 -b~3-e are K8Ta6O 19 , CoPc and CoT-7 XPS high-resolution spectra. It can be seen that in the full XPS spectrum of CoT-7, K, Ta, C, N, O and Co elements are present, which once again shows that the composite catalyst has been successfully obtained ( Figure 3 -a). Figure 3 -b is the high-resolution XPS spectrum of Co 2p, the signal peaks at 780.8eV and 796.5eV are Co 2p 3 / 2 and Co 2p 1 / 2 orbitals, indicating that the cobalt ion in cobalt phthalocyanine is divalent. Compared with pure CoPc, the peak of Co 2p in CoT-7 shifts slightly toward the low binding energy direction, which is caused by K8Ta6O 19 Produced by electron transfer between CoPc. Figure 3 -c, the peaks at 25.8 and 27.8 eV are 4f of tantalum 7 / 2 and 4f 5 / 2 orbital, indicating that its valence state is +5. Figure 3 -d is the O1s high-resolution XPS spectrum for K8Ta6O 19 The peaks at 530.2, 531.5, and 532.7 eV correspond to lattice oxygen, oxygen vacancies, and surface adsorbed OH species, respectively, which are consistent with the original K8Ta6O 19 In comparison, the proportion of oxygen vacancies in CoT-7 is significantly increased. Figure 3 -e is the N1s spectrum. Compared with pure CoPc, the N1s peak of CoT-7 shifts to a lower binding energy by 0.5 eV. In summary, compared with the two pure components, the binding energies of Co and N elements in CoT-7 shift to a lower direction, while the binding energies of Ta and O elements shift to a higher energy direction. This is due to the interaction between CoPc and K8Ta6O 19 The interaction between them was caused, and it also indicated the successful construction of CoT-7.

[0039] Figure 4 K8Ta6O 19 and CoPc electron paramagnetic resonance spectra (EPR). As can be seen from the figure, the K8Ta6O 19 After being introduced into CoPc, the same as pure K8Ta6O 19 In comparison, the oxygen vacancy signal is enhanced, indicating that the CoT-7 heterojunction exhibits an increasing trend in oxygen vacancy concentration. Abundant oxygen vacancy defects favor CO2 activation and lower the reaction energy barrier. Therefore, in this example, the increased oxygen vacancy concentration promotes CO2 adsorption and activation, inhibits the recombination of photogenerated electrons and holes, and thus improves the photoelectrocatalytic performance of the catalyst.

[0040] Figure 5 CoPc, K8Ta6O 19 And UV-visible absorption spectra and fluorescence spectra of the composite catalyst CoT-X. Figure 5 -a and 5-b are CoPc, K8Ta6O 19 and the UV-visible absorption spectra of CoT-X, Figure 5 -c is K8Ta6O 19 and CoT-X fluorescence spectra. It can be seen that pure K8Ta6O 19 There is a sharp absorption peak at 252nm in the ultraviolet region ( Figure 5 -a). Due to its conjugated macrocyclic structure, cobalt phthalocyanine has two distinct characteristic peaks, namely B band (250-350nm) and Q band (600-700nm) ( Figure 5 -b), indicating the formation of cobalt phthalocyanine. At the same time, K8Ta6O 19 The sharp absorption peaks of cobalt phthalocyanine can also be observed. Figure 5 -a). It is worth noting that compared with CoPc, the Q band of CoT-X has a red shift and the B band has a blue shift, indicating that there is electron transfer between the two components. Figure 5 -c can be seen that the same pure K8Ta6O 19 In comparison, the composite catalysts CoT-X all exhibited smaller fluorescence peak intensities, indicating that the introduction of cobalt phthalocyanine can effectively reduce the electron-hole recombination rate and increase the carrier mobility rate. Among them, CoT-7 had the lowest fluorescence peak intensity, indicating that it had the lowest carrier recombination rate. This is due to the formation of the heterojunction between the two semiconductors, which formed a built-in electric field and accelerated the separation of electrons and holes, which will be beneficial to the improvement of catalytic performance.

[0041] Figure 6 For different K8Ta6O 19 Electrochemical characterization of the added amount of composite catalyst CoT-X. Figure 6 -a is the LSV curve of CoT-X in dark CO2 atmosphere. It can be seen that different K8Ta6O 19 The composite catalysts with different addition amounts all have large current densities, among which CoT-7 has the largest current density. The current density increases significantly under light irradiation, and the increase mainly occurs in the bias range of -0.6 to -1.0 V vs RHE ( Figure 6 -b) This is the highest FE of CoT-7 CO This phenomenon indicates that light irradiation only has a relatively significant effect on CO2 reduction, but has no significant effect on hydrogen evolution. 19 Compared with CoPc illumination conditions, CoT-7 has a significantly improved photocurrent density, which is attributed to the formation of heterojunction. Figure 6 -c and 6-d are CoPc, K8Ta6O 19 and CoT-X impedance spectra and transient photocurrent response curves. 19 The synergistic effect of CoT-X, CoPc and K8Ta6O 19 Compared with the above samples, CoT-7 shows higher current density and smaller interface charge transfer impedance, among which CoT-7 has the highest current density and the smallest impedance, which is consistent with the LSV test results. Figure 6 -e is K8Ta6O 19 , CoPc and CoT-7 Tafel slope, it can be seen that under light conditions, the Tafel slope of CoT-7 (120mVdec -1 ) is much lower than CoPc (265mV dec -1 ) and K8Ta6O 19 (157mV dec -1 ), indicating that it has more favorable kinetic characteristics in the photoelectrocatalytic CO2 reduction process. At the same time, the slope value is significantly lower than that under dark conditions (137mV dec -1 ), indicating that light is beneficial to improve the charge transfer kinetics of electrocatalytic CO2 reduction. In order to further explore the potential influencing factors of the excellent catalytic performance of CoT-7, the electrochemical double layer capacitance (Cdl) of the composite catalyst was calculated based on the cyclic voltammetry curve to estimate the electrochemical active surface area (ECSA). Figure 6 -f, the Cdl of the CoT-7 heterojunction is 1111 μF cm -2 , significantly better than CoPc (582μF cm -2 ) and K8Ta6O 19 (697 μF cm -2), indicating that the CoT-7 heterojunction has a larger ECSA, which can provide more active sites for photoelectrocatalytic CO2 reduction, making it have the best activity.

[0042] (2) Photoelectric performance test:

[0043] 8 mg of the prepared catalyst CoT-X (Example 1) or CoN-7 (Comparative Example 3), 3.5 mg of carbon black, 240 μL of high-purity water, 240 μL of isopropanol, and 160 μL of 5% Nafion solution were stirred to form a catalyst suspension. The suspension was then ultrasonicated for 2 h and evenly dropped onto hydrophobic carbon paper with an area of ​​approximately 1 × 2 cm. 2 The catalyst loading was controlled at 0.35 mg cm -2 Then, vacuum dry at 60°C.

[0044] The photoelectric performance of all catalysts was tested using an H-type electrolytic cell on an electrochemical workstation (CHI760E). A Pt sheet electrode and an Ag / AgCl electrode served as the counter and reference electrodes, respectively, and the catalyst served as the working electrode. A 300W xenon lamp was used for illumination. For CO2 photoelectrochemical reduction performance testing, 0.5M KHCO3 was used as the electrolyte. Prior to the reaction, the electrolyte was aerated with CO2 for 20 minutes. Chronoamperometry was used for measurement at each fixed potential, and the resulting gaseous products were detected by gas chromatography.

[0045] Figure 7 The photoelectrocatalytic CO2 reduction test results of different composite catalysts. Figure 7 -a-7-c are different K8Ta6O 19 The Faraday efficiency (FE CO ); Figure 7 -d is the Faradaic efficiency of CoPc and CoN-7; Figure 7 -e is K8Ta6O 19 , CO current density of CoPc and CoT-7 at different applied potentials; Figure 7 -f is the CO yield of CoPc and CoT-7; Figure 7 -g is the CO yield of PC, EC, PC+EC and PEC-CO2RR on CoT-7; Figure 7 -h is the TOF value of CoPc and CoT-7.

[0046] like Figure 7 -a, in different K8Ta6O 19 The FE of CoT-7 was CO Higher than the other two ratios, at -0.9V vs.RHE, FE COAs high as 99.5%, indicating that CoPc and K8Ta6O 19 The optimal ratio is 1:7, and nuclear magnetic resonance detection shows that no liquid product is generated in the reaction system.

[0047] Taking the best catalyst CoT-7 as a representative, its photoelectrocatalytic CO2RR performance was further studied. Figure 7 -b, K8Ta6O 19 No CO is generated during the reaction (FE CO =0%), indicating that K8Ta6O 19 It is not an active center for CO2 reduction, but the formed CoT-7 shows obvious photoelectrocatalytic CO2RR performance, and its Faradaic efficiency is further improved under light conditions, indicating that under the drive of light, CoT-7 can generate more electrons to participate in the CO2 reduction reaction and inhibit the hydrogen evolution reaction. Figure 7 -c, when the heating temperature of the catalyst is increased to 350 ° C, the FE of CoT-7 obtained CO =0, indicating that when the temperature rises, the catalyst structure is destroyed and can no longer catalyze the reduction of carbon dioxide. Figure 7 -d, using K7HNb6O 19 Replace K8Ta6O 19 The catalyst CoN-7 heterojunction prepared according to the optimal ratio also showed good CO2 reduction performance under dark and light conditions, but slightly lower than CoT-7. This is because K8Ta6O 19 The superior electron-donating ability makes the photoelectrocatalytic CO2 reduction performance of CoT-7 better than that of CoN-7.

[0048] The current density of carbon monoxide (j CO ) is also an important factor in evaluating the catalytic performance of electrocatalysts. Figure 7 -e, K8Ta6O 19 No photoelectrocatalytic CO2RR activity, its j CO Compared with pure CoPc, under light conditions, the j CO (25mAcm -2 , -0.9V vs.RHE) than CoPc (10.1mA cm -2 , -0.9 V vs. RHE) increased by 2.5 times, indicating that the introduced K8Ta6O 19 There is a strong synergistic effect between CoT-7 and CoPc, and the j CO There is a certain improvement, which shows that the external light field is beneficial to promoting the electron transfer ability of CoT-7. Figure 7-f also showed similar results. Under the photoelectrocatalytic conditions, CoT-7 showed a significant CO yield (245.22 μmol cm -2 h -1 , -0.9 V vs. RHE), and increases with the increase of external bias voltage, reaching 309.65 μmol cm at -1.0 V vs. RHE. -2 h -1 , is the CO yield of CoPc under the same conditions (91.29 μmol cm -2 h -1 , -0.9V vs. RHE) is 3.4 times.

[0049] In order to further demonstrate the optoelectronic synergy of the CoT-7 heterojunction, as Figure 7 As shown in Figure 5-g, the CO yield under single photocatalytic conditions and electrocatalytic conditions (i.e., dark state) is compared. It can be seen that the CO yield under photoelectric synergy is not a simple sum of photocatalysis and electrocatalysis, indicating the synergistic effect between PC and EC.

[0050] In order to better evaluate the catalytic performance of the catalyst, the turnover frequency (TOF) of CO was calculated based on the Co atomic content. CO ).like Figure 7 -h shows the TOF of CoT-7 heterojunction under illumination conditions. CO Much higher than CoPc, and significantly better than the TOF value under dark conditions. At -1.0V vs. RHE, it can reach up to 4661h -1 This fully demonstrates that the heterojunction structure can maximize the exposed active surface sites, thereby achieving the highest TOF CO .

[0051] Figure 8 The stability test results of the composite catalyst CoT-7 are shown. It can be seen that during the 12-hour test, the current density of CoT-7 did not change significantly, nor did the Faradaic efficiency decrease significantly. It still maintained a high CO selectivity of over 95%, demonstrating the excellent stability of the prepared catalyst.

[0052] (3) Research on photoelectrocatalytic mechanism

[0053] Figure 9 CoPc and K8Ta6O 19 The band gap width and Mott-Schottky diagram of CoPc and K8Ta6O were first calculated to study the photoelectrocatalytic mechanism of the system. 19 The band gap width, such as Figure 9-a, which are 1.26eV and 4.65eV respectively. In order to give the band structure of the catalyst, CoPc and K8Ta6O were tested at 500, 800 and 1000 Hz respectively. 19 The Mott-Schottky diagram of Figure 9 -b and 9-c. CoPc and K8Ta6O 19 Both show positive slopes, indicating that they are n-type semiconductors, and their flat band potentials are -1.5 V and -0.58 V (relative to the standard hydrogen electrode), respectively. From this, it can be estimated that the CB and VB of CoPc are -1.6 V and -0.34 V (vs. NHE), respectively. 19 The CB and VB are -0.68V and +3.97V respectively (vs. NHE).

[0054] Figure 10 The photoelectrocatalytic mechanism of the composite catalyst of the present invention is given by CoPc and K8Ta6O 19 The matching band gap, combined with the above performance tests, suggests that the system is a Z-type heterojunction electron transfer mechanism. 19 The Fermi level of CoPc is relatively low and can be used as an oxidative photocatalyst, while the Fermi level of CoPc is relatively high and can be used as a reducing photocatalyst. When two semiconductors with different Fermi levels come into contact with each other, a new internal electric field is formed. Therefore, after contact, according to the energy band structure of the two semiconductors, electrons will be transferred from K8Ta6O to CoPc. 19 Migrate to CoPc to achieve a new Fermi level balance, making K8Ta6O 19 The interface between K8Ta6O and CoPc forms a strong donor-acceptor coupling. Due to the redistribution of free electrons, the surface of CoPc becomes positively charged, while K8Ta6O 19 The surface of K8Ta6O becomes electronegative, thus forming a unique built-in electric field at the interface. When CoT-7 is excited by light, under the synergistic effect of the built-in electric field and Coulomb interaction, K8Ta6O 19 The photogenerated electrons on CB will recombine with the photogenerated holes on CoPcVB, resulting in excess electrons in the CB of CoPc, which react with CO2 to generate CO, while the oxidation reaction occurs on K8Ta6O 19 On VB.

[0055] In summary, the present invention prepared a Z-type heterojunction composite catalyst by an impregnation-calcination method, and introduced K8Ta6O into cobalt phthalocyanine. 19This is beneficial to improving the stability of the material and inhibiting the agglomeration of cobalt phthalocyanine during the catalytic process. Among all the prepared composite catalysts CoT-X, CoT-7 has the best photoelectrocatalytic carbon dioxide reduction performance, and can highly selectively convert CO2 into CO with a Faradaic efficiency of up to 99.5% and a TOF value of up to 4661.6h -1 (-1.0V vs. RHE) and excellent stability, lasting for more than 12 hours. UV-visible absorption spectroscopy, Mott-Schottky measurements, and a series of experiments demonstrated that this system exhibits a Z-type heterojunction electron transport mechanism, significantly improving the separation efficiency of photogenerated carriers.

[0056] The embodiments described above are only preferred embodiments of the present invention and are only used to explain the present invention, not to limit the scope of implementation of the present invention. For those skilled in the art, it is of course possible to easily make other implementation methods by replacing or changing the technical content disclosed in this specification. Therefore, all changes and improvements made on the principles of the present invention should be included in the scope of the patent application of the present invention.

Claims

1. A method for preparing a Z-type heterojunction composite catalyst, characterized in that: The method comprises the following steps: dispersing potassium hexatantalate in an anhydrous ethanol solution of cobalt phthalocyanine, stirring, ultrasonicating, evaporating the anhydrous ethanol, and then calcining the obtained solid product at 300°C for 2h to obtain K8Ta6O 19 / CoPc composite catalyst.

2. The preparation method according to claim 1, characterized in that The ratio of the cobalt phthalocyanine to potassium hexatantalate is 1:6-8.

3. The preparation method according to claim 1, characterized in that The usage ratio of anhydrous ethanol to cobalt phthalocyanine is 40 mL:0.025 g.

4. The preparation method according to claim 1, characterized in that The evaporation temperature of the anhydrous ethanol is 95°C.

5. Use of the Z-type heterojunction composite catalyst prepared by the preparation method according to any one of claims 1 to 4 in a photoelectrocatalytic CO2 reduction reaction.

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