Shuai Fan, Huiyuan Cheng, Manman Feng, Xuemei Wu, Zihao Fan, Dongwei Pan, Gaohong He

State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

Keywords:Carbon dioxide Electrochemistry Selective catalytic reduction Electrochemical hydrogen pump Nitrogen-doping carbonized ZIF-8

A B S T R A C T The conversion of CO2 electrocatalytic hydrogenation into energy-rich fuel is considered to be the most effective way to carbon recycle.Nitrogen-doping carbonized ZIF-8 is proposed as carrier of the earth-rich Sn catalyst to overcome the limit of electron transfer and CO2 adsorption capacity of Sn. Hierarchically porous structure of Sn doped carbonized ZIF-8 is controlled by hydrothermal and carbonization conditions, which induces much higher specific surface area than that of the commercial Sn nanoparticle(1003.174 vs.7.410 m2·g-1).The shift of nitrogen peaks in X-ray Photoelectron Spectroscopy spectra indicates interaction between ZIF-8 and Sn,which induces the shift of electron cloud from Sn to the chemical nitrogen to enhance conductivity and regulate electron transfer from catalyst to CO2.Lower mass transfer resistance and Warburg resistance are investigated through EIS, which significantly improves the catalytic activity for CO2 reduction reaction (CO2RR). Onset potential of the reaction is reduced from-0.74 V to less than -0.54 V vs. RHE. The total Faraday efficiency of HCOOH and CO reaches 68.9% at-1.14 V vs. RHE, which is much higher than that of the commercial Sn (45.0%) and some other Sn-based catalyst reported in the literature.

1. Introduction

Large-scale utilization of coal, oil and natural gas has brought about serious problems as resource shortage, air pollution and‘‘greenhouse effect” [1,2]. It is of great significance to convert CO2into useful energy or chemical intermediates, by which both‘‘green chemistry” and ‘‘a(chǎn)tomic economy” could be achieved to greatly help the recycle of fossil fuels [3-5]. Compared with high-temperature heterogeneous catalysis, photocatalytic reduction and other CO2reduction technologies [6,7], electrocatalytic hydrogenation has attracted more and more attention, owing to a higher reaction rate, easy to scale up, and more practical and potential industrial value [8,9].

Development of highly efficient and selective electrocatalysts is a key to the thermodynamically stable CO2molecule. The earthrich Sn catalyst is widely used because of its high hydrogen evolution potential, high HCOOH selectivity, low price, environmental protection and non-toxicity [10-13]. The structure, conductivity and CO2adsorption performance of the electrode directly affect the hydrogenation efficiency. Since the electron activation of CO2is the rate-controlling step of CO2RR,improving the electron transport capacity of Sn catalyst supports has become a research hotspot. For example, Prakaet al.loaded Sn onto gas diffusion layer to enlarge three-phase interface and obtained hydrogenation product HCOOH with Faraday efficiency close to 70% [14]. Wanget al.hot-pressed PTFE, carbon black, and copper mesh deposited with Sn to tune the porosity for gas diffusion [15,16]. Irtemet al.deposited Sn particles on cathode side carbon paper to improve the conductivity of the catalytic layer and reduce the reaction potential [17]. Leiet al.performed nitrogen doping treatment on carbon nanotubes to change the local charge density,improve electron transport, the HCOOH Faraday efficiency reaches 46% at-1.3 Vvs.Ag/AgCl. Moreover, the N atoms on the walls of nitrogen-doped carbon nanotubes have negative charges.The electric charge can be combined with the CO2in the solution without surface treatment such as acid treatment to promote the hydrogenation reaction[18].Metal-organic framework materials(MOFs)are effective catalyst carrier owing to special porosity,high specific surface area and ligand designability. Yanget al.combined porphyrin MOF with Co,and the Faraday efficiency of CO was achieved more than 50% [19]. The preparation of Cu and Fe-based catalysts with ZIF-8 as a carrier or template shows high catalytic activity for CO2RR. The doping of heteroatoms (such as N, P and S) can change the electronic state of metal elements and improve their electron conductivity and catalytic activity [20]. Bianet al.prepared cube-shaped and star-shaped octahedral Sn-MOF, which showed excellent cycle stability when used in lithium batteries through a large number of tentative experiments [21]. Mainaet al.[22,23] synthesized Cu-TiO2doped ZIF-8 catalyst, and the selectivity of methanol as a product of photocatalytic reduction of CO2reached 70%. Guoet al.[24] prepared Fe2O3/ZIF-8 catalysts with different sizes, showing good selectivity of low olefin and stable at 300 °C and 3 MPa. In conclusion, MOFs have N doping,special porosity,high specific surface area and ligand designability,and the interaction between N and metal catalysts is reported to conducive to the hydrogenation reaction.However,there is no carbonization process for MOFs at high temperature used as catalyst carrier, and the electrical conductivity is generally poor, and the synergistic catalytic effect between MOFs and Sn needs to be studied systematically.

Herein,carbonized N doping ZIF-8 porous carbon is proposed as carrier of Sn catalyst to significantly improve electron transport,specific surface area and synergistically catalyze CO2RR. By optimizing the hydrothermal synthesis conditions and carbonization temperature, the micro and mesoporous carbonized ZIF-8 with high specific surface area had prepared to enhance CO2adsorption.XPS and AC impedance analysis show that ZIF-8-800 interacts with Sn to form chemical nitrogen, which has a catalytic effect, while reducing the ohmic resistance,charge transfer resistance and diffusion resistance of CO2mass transfer.After doping with 20%(mass)ZIF-8-800, the Sn catalyst showed higher CO2electrocatalytic activity, and the onset potential is reduced to -0.54 Vvs.RHE.Compared with commercial Sn catalyst, the formation rates of hydrogenation products HCOOH and CO increased by 68.7% and 109.3%, respectively; Faraday efficiency increased by 43.6% and 70.4%, respectively, and the Faraday efficiency of HCOOH is in the high level among those reported in the literature.

2. Materials and Methods

2.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 2-methylimidazole (2-MeIM) were obtained from Aladdin. Potassium bicarbonate (KHCO3) and Isopropanol (C3H8O) were purchased from Tianjin FuChen Chemical Reagent, China. Ethanol was acquired from Tianjin TianLi Chemical Reagent,China.Hydrogen peroxide solution (H2O2) and concentrated sulfuric acid(H2SO4) was obtained from Tianjin Kemeo Chemical Reagent,China. 5% (mass) Nafion solution (Dupont) were purchased from Shanghai Hephas Energy Equipment, China. Commercial 70% Pt/C obtained from Dalian Xinyuan Dongli, China. Nafion 115 obtained from DuPont,America.Carbon paper(AvCarb MB30)acquired from Ballard, Canada. Nano Sn powder (50 nm) purchased from Shanghai ChaoWei Nano, China. H2, CO2, N2(high purity) were acquired from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China.

2.2. Preparation of ZIF-8 and its carbides

Preparation of ZIF-8:ZIF-8 was prepared by a typical solvothermal method, 0.800 g (2.66 mmol) of Zn(NO3)2·6H2O and 1.765 g(21.28 mmol) of 2-MeIM were dissolved in 20 ml of deionized water respectively. After stirring for 20 min, the uniform zinc nitrate solution was slowly added dropwise to the 2-methylimidazole solution, and the mixed solution quickly turned into milky white.The mixture was kept at room temperature with agitating for 4 h.Next,the white precipitate named ZIF-8 was collected by centrifugation (10,000 r·min-1, 8 min) and thoroughly washed with deionized water and methanol for three times, and dried at 80 °C under vacuum overnight.

Preparation of ZIF-8-T: Disperse a certain amount of ZIF-8 particles evenly in the magnetic boat and pyrolysis in the tube furnace, heated under certain temperature (600, 700, 800, 900 °C)for 3 h at a heating rate of 5 °C·min-1under Ar atmosphere. Dissolve the prepared black powder product in a 0.5 mol·L-1H2SO4solution for 6 h at 80°C to remove inorganic impurities which possible exist. The obtained sample is collected by a high-speed centrifuge, washed with distilled water three times, centrifuged and filtered, and washed to neutral with a large amount of distilled water.Drying in a vacuum oven at 80°C to obtain the final porous carbon material,which marked ZIF-8-T(T=600°C,700°C,800°C,900 °C).

2.3. Physical characterization

The scanning electron microscopy (SEM) test was used the NOVA NanoSEM NPE218 (FEI, USA) field emission scanning electron microscope to characterize the surface morphology and characteristics of ZIF-8 and its carbide ZIF-8-T, and high vacuum ion sputtering coating apparatus (O150TES) was used to spray gold,which to conduct electricity before the sample test.

The X-ray diffraction (XRD) test was analyzed by D/MAX-2400(JEOL, Japan) X-ray diffractometer, and the equipment parameters were as follows: CuKα radiation (wavelength λ = 0.15406 nm) is used as the light source, the tube voltage was 40 kV, tube current was 100 mA,the scanning speed was 5(°)·min-1,scanning step was 0.01°, and the scanning range was 2θ = 5°-50°.

The nitrogen adsorption/desorption isotherms were measured by Autosorb-1-MP (Quanta, USA). The sample was pretreated under vacuum at 120 °C for 12 h before test, and specific surface area was calculated through the BET method model. The nonlocal density functional theory (NLDFT) was used to calculate the pore size distribution.

X-ray photoelectron spectroscopy (XPS) was a technical means to study the surface and surface process of solid materials.Detailed surface chemical environment was carried out with X-ray photoelectron spectroscopy (XPS) on ESCALAB 250Xi, Al Kα X-ray was adopted as exciting source. X-ray photons are used to excite the inner electrons of atoms on the surface of a substance, and the XPS energy spectrum is obtained by energy analysis of these electrons. It can be used to analyze the chemical composition of the sample surface or determine the chemical state of each element.Generally, XPS can measure all elements except H and He.

2.4. Electrochemical activity of carbonized ZIF-8 doped with Sn catalyst

Electrochemical impedance spectroscopy (EIS) tests were carried out by using CHI1140 electrochemical workstation (Chenhua,China).The EIS test was to apply a sine wave AC signal disturbance to the reactor, observed the corresponding response signal generated by the reactor system, and obtain the impedance of the electrode by analyzing the signal.A series of sine wave signals changed from high frequency to low frequency leads to the generation of an impedance spectrum, which is called electrochemical impedance spectroscopy.Through the equivalent circuit analysis,it is possible to specifically analyze the internal resistance of the system and the transmission resistance of electrons from the catalyst surface to the reaction interface. The above parameters can be used to characterize and evaluate the advantages and disadvantages of the reaction system.The specific parameter settings in this study were as follows:set the high frequency to 106Hz and the low frequency to 0.1 Hz under the open circuit voltage, and the frequency decreased from high frequency to low frequency,recorded the corresponding signal value, and fitted the series-parallel circuit;drawn the Nyquist curve with the real partZ’of the complex impedanceZas the abscissa,and the opposite number of the imaginary part -Z‘‘ as the ordinate.

The linear sweep voltammetry test was carried out by the CHI1140 electrochemical workstation (Chenhua, China), which reflected the corresponding current change within a certain range of voltage change,so as to indicate the voltage range and degree of the redox reaction of the substance.In this study,the voltage range was generally set at 0.2 V to-1.5 Vvs.RHE,the speed is 0.05 V·s-1,scanned for one circle.The reduction reaction in this voltage range was analyzed based on the potential required for CO2RR process in this voltage range, so as to avoid corrosion caused by excessive voltage of graphite plate.The LSV curve performs a one-way reduction reaction test. The specific operation was as follows:30 ml·min-1of H2into the anode, and 20 ml·min-1of N2or CO2into the cathode. When N2was passed on the cathode side, due to the lack of hydrogenation reactants, only hydrogen evolution reaction occurred, and when CO2was passed, hydrogenation and hydrogen evolution reactions occurred at the same time. By analyzing the current difference between them, the degree of hydrogenation reaction and the onset potential could be calculated and judged.

2.5. CO2 electroreduction hydrogenation experiment process and product analysis evaluation index

The electrochemical hydrogen pump (EHP) was used for CO2hydrogenation, and the schematic of EHP was shown in Fig. 1(a).The EHP reactor had the same configuration as a proton exchange membrane fuel cell, which had serpentine flow channels with an effective reaction area of 5.29 cm2for both the anode and cathode.Nafion 115 (Dupont) anion-exchange membrane was prepared to make membrane electrode assembly (MEA) with catalyst loading 0.5 mg·cm-2Pt/C by hot pressing at 140 °C for 90 s, which served as a hydrogen oxidation anode. The flow rates of H2and CO2through anode and cathode gas diffusion layers were controlled by mass flowmeter to be 30 ml·min-1and 20 ml·min-1, respectively. The catalyst loading of commercial nano Sn powder was 2 mg·cm-1, and the doping amount of ZIF-8-T was 10% (mass),20% (mass), and 30% (mass) of the catalyst Sn powder. The buffer solution (30 ml 0.5 mol·L-1KHCO3) was circulated in the buffer layer through a peristaltic pump with a flow rate of 15 ml·min-1.The Ag/AgCl reference electrode was placed in the middle of buffer layer and connected to electrochemical workstation with working electrode and counter electrode.The reacted H2was discharged to the outside,and the CO2outlet pipe was inserted into the buffer solution (to prevent liquid products flowing out with gas), and then connected to the gas chromatograph for on-line detection of the products.

Fig.1. Experimental procedure of CO2RR in EHP and product detection spectrum.(a)Reaction device diagram,(b)Nuclear Magnetic Resonance hydrogen spectrum 1H NMR,(c) Gas Chromatography on-line detection spectrum.

The main product of electrochemical hydrogen pump CO2hydrogenation was liquid formic acid (HCOOH), and a small amount of gas phase carbon monoxide(CO)and by-product hydrogen (H2) were also generated.

The liquid phase product HCOOH was detected by1H NMR,which is a very sensitive detection method for hydrocarbons. In the nuclear magnetic resonance hydrogen spectrum,the hydrogen in different chemical environments in the molecule showed different chemical shifts,that is,the peak position,the number of peaks was the number of the chemical environment of hydrogen;and the ratio of the peak area was the corresponding ratio of the number of hydrogen atoms in different chemical environments. In the experiment,the HCOOH solution generated by the cathode reaction was circulated into the buffer tank along with the buffer solution, and the cathode outlet pipe was also inserted below the liquid level of the buffer tank to prevent the liquid phase product from being discharged along with the gas path. In order to detect the amount of HCOOH, a known concentration of 0.11 mg · L-1DMSO internal standard solution was added to the NMR tube, and D2O was used as the solvent for NMR detection, as shown in Fig. 1(b). The MestReNova software, which analyzes nuclear magnetic resonance spectra,was used to automatically integrate the area of each peak,and the ratio of the peak area of the characteristic peak of HCOOH and DMSO was obtained. The standard curve of ‘‘peak area ratioamount of HCOOH” made with a known amount of DMSO was brought into it to obtain the amount of HCOOH produced in the reaction.

The gas products were detected by gas chromatography, and inert gas was used as the mobile phase. CO and H2can separation and detection because of the different adsorption capacity of each component by using the chromatographic column.The cathode gas outlet pipeline of the EHP reactor was directly connected to the gas chromatograph inlet to realize on-line monitoring, as shown in Fig. 1(c). The detectors were FID and TCD. TCD was used to detect high content of CO and H2, and a methane reformer was installed in the FID channel to detect low CO content (less than 583.57 mg · m-3). The temperature of chromatographic column and methane reformer were 65 °C and 350 °C, respectively; and the temperature of FID and TCD detector were both 150 °C.

During the experiment, analytical instruments can record the peak area of the instantaneous gas phase product. The detected gas peak area was compared with the standard gas peak area to obtain the ratio of the detected gas content to the known content,that is, the single-point correction method was used to calculate the detected gas content.Before sampling,the gas flow in the outlet pipe of the reactor was measured and recorded with a soap bubble flowmeter.

The evaluation indicators of hydrogenation performance mainly include Faradaic efficiency and reaction rate. Faradaic efficiency is defined as the percentage of electrons consumed to produce the target product, which can be understood as the selectivity of the target product.

The reaction rate is usually obtained by dividing the total current by the geometric area of the working electrode. The reaction rate of a specific product can be obtained from the product of the total current density and its current efficiency. According to formula (2), the reaction rate of a specific product has a linear relationship with its partial current density.

wheremproductis the number of moles of a specific product,jproductrepresents the partial current density of the specific product,nindicates the number of transferred electrons,Fdenotes the Faraday constant(96485.34 C·mol-1),Iis the detection current value,trepresents the reaction time,Aindicates the effective geometric area of the working electrode, which is 5.29 cm2.

3. Results and Discussion

3.1. Structural characterization of the carbonized ZIF-8

The morphologies of ZIF-8 crystals prepared under different reaction conditions are shown in Fig. 2. Investigations of different reaction times, reaction temperatures and reaction solvents show that 4 h reaction with H2O solvent at 25°C is the best preparation conditions,as shown in Fig.2(b).The crystal morphology is incomplete with less reaction time (Fig. 2(a)) in the initial nucleation stage. The crystal particle size is larger than 200 nm with more reaction time (Fig. 2(c)), and the larger particle size is not suitable as a catalyst carrier in subsequent experiments.When the reaction temperature rose to 60 °C (Fig. 2(d)), the produced crystals aggregate with irregular morphology.The reaction conditions in Fig.2(b)are better. On this basis, the reaction time is selected as 4 h. The reaction solvent is also explored,as shown in Fig.2(e)(f).The crystal particle size is smaller size in 4 h reaction time when changed the reaction solvent to CH3OH(Fig.2(f)),while the size is different and has poor structural stability. When the temperature rises to 60 °C (Fig. 2(e)), CH3OH is still used as solvent, the particle size is significantly increased compared to Fig.2(f)which reaction temperature is 25°C,and the crystal particle size is uniform while the repeatability is poor. In summary, the best ZIF-8 preparation conditions are the 4 h reaction with H2O as the solvent at 25 °C in Fig. 2(b).

Fig.2. SEM images of ZIF-8 under different preparation conditions,the reaction time is 2 h,4 h,6 h,the reaction temperature is 25°C,60°C,and the solvent is H2O or CH3OH.(a) H2O 25 °C 2 h, (b) H2O 25 °C 4 h, (c) H2O 25 °C 6 h, (d) H2O 60 °C 4 h, (e) CH3OH 60 °C 4 h, (f) CH3OH 25 °C 4 h.

The carbonization temperature had a great impact on the morphology of the ZIF-8-Tcrystal (Tis the carbonization temperature,T= 600 °C, 700 °C, 800 °C, 900 °C), as shown in Fig. 3. The SEM image of uncarbonized ZIF-8 as shown in Fig.3(a),The particle size is between 140 and 160 nm as shown in the SEM image of uncarbonized ZIF-8 (Fig. 3(a)). The crystal structure of ZIF-8-600 is partially agglomerated with low carbonization temperature as carbonization degree incomplete (Fig. 2(b)). The ZnO produced by ZIF-8-900 skeleton decomposition is reduced with high carbonization temperature(Fig.3(e)),owing to the Zn metal evaporated at high temperature (Zn boiling point is 907 °C) or the carbonized material is vaporized, which causing the crystal structure collapse.The crystal structure of ZIF-8-800 is the clearest and most complete with the particle size changed between 110-130 nm(Fig. 2(d)), slightly smaller than the 120-140 nm of ZIF-8-700(Fig. 3(c)), and there is no obvious agglomeration and collapse,which is the optimal temperature carbonization temperature to keep the carbide crystal structure. The surface of the ZIF-8-800 carbide contains a large amount of C,N,and Zn elements as shown in SEM-EDS energy spectrum (Fig. 3(f)), and the N element is highly uniformly dispersed in the carbon skeleton of ZIF-8-800 carbide. The heteroatoms doped in porous carbon materials (such as boron, nitrogen, sulfur, and phosphorus) have a great influence on their physical and chemical properties. Among them, the doping of nitrogen atoms has been favored by a large number of researchers. The first reason is that the lone pair electrons of the nitrogen atom can increase the charge density around the porous carbon material as carriers, so as to improve the conductivity of the porous carbon material [25]. The second is that the incorporation of nitrogen atoms increases the defect sites, which also makes the number of active sites in the porous carbon materials increased. The third is the existence of nitrogen atoms can increase the affinity and biocompatibility of porous carbon materials as electrode materials with electrolytes. Fourth, the presence of nitrogen atoms can improve the binding capacity of porous carbon materials and metal ions, and improve the loading capacity of metals on the surface of porous carbon materials. The N element in ZIF-8-800 carbide is highly uniformly dispersed,which is helpful to improve the conductivity and catalytic activity in the CO2catalytic hydrogenation system.

Fig.3. Schematic illustration of ZIF-8 and ZIF-8-T.(a)SEM image of ZIF-8,(b)SEM image of ZIF-8-600,(c)SEM image of ZIF-8-700,(d)SEM image of ZIF-8-800,(e)SEM image of ZIF-8-900, and (f) SEM-EDS mapping of ZIF-8-800.

The X-ray diffraction (XRD) spectra of ZIF-8 and carbonized ZIF-8 (denoted as ZIF-8-carbonized temperatureT) show that the prepared samples are pure phase ZIF-8 with high crystallinity,as shown in Fig. 4. ZIF-8 and ZIF-8-Tare nanoporous materials with zeolite topological structures. Typical ZIF-8 characteristic peaks are shown in samples with different carbonization temperatures, and the positions of (0 1 1), (0 0 2), (1 1 2), (0 2 2),(0 1 3), (2 2 2) diffraction peak positions are consistent with those reported in the literature [26]. When the carbonization temperature reaches 900 °C, the characteristic peak of ZIF-8 has completely disappeared, ZIF-8 has been violently decomposed and carbonized, and Zn is reduced by C to metallic Zn and discharged with Ar gas flow, which has certain characteristics of graphite phase.

The nitrogen-doped ZIF-8-800 exhibits much higher specific surface area than that of the commercial Sn nanoparticle,as shown in the N2adsorption-desorption isotherm in Fig. 5(a). It has the characteristics of Ⅳ isotherm with specific surface area(Brunauer-Emmett-Teller BET method) of about 1003.174 m2·g-1,while the specific surface area of the commercial Sn is only about 7.410 m2·g-1. It is attributed to the hollow structure and fine particle stacking morphology. As shown in Fig. 5(b), the nitrogendoped nanoporous ZIF-8-800 shows micropore(＜2 nm)and mesopores (3.6-4.2 nm), which are derived from thermally induced defect[27]and contribute to 844.600 m2·g-1and total pore volume of about 1.025 cm3·g-1.The average size of the micropores is about 0.57 nm, slightly larger than that of the CO2kinetic diameter of 0.33 nm, and the high density of active metal sites in the nitrogen-doped nanoporous material provides additional adsorption sites for CO2molecules. Micro and mesopores in can increase the specific surface area and enhance the adsorption of CO2gas,which is conducive to improve the reaction rate of CO2catalytic hydrogenation.

Fig. 4. XRD patterns of ZIF-8 and ZIF-8-T (T = 600, 700, 800, 900 °C).

Fig. 5. Nitrogen adsorption-desorption isotherms. (a) N2 sorption/desorption curves and (b) NLDFT pore size distribution plot of and ZIF-8-800 and commercial Sn.

Fig. 6. XPS spectra of ZIF-8 and commercial Sn powder doped with 20%(mass)ZIF-8-800. (a) N1s spectra, (b) Sn 3d spectra.

As 20%(mass)ZIF-8-800 doped with commercial Sn(denoted as Sn-ZIF-8-800-20%),the shift of nitrogen peaks in XPS spectra indicates interaction between ZIF-8-800 and Sn,are shown in Fig.6(a).Nitrogen existed in the form of structural nitrogen and chemical nitrogen in carbonized ZIF-8-800 [28]. The peak positions of pyridinic nitrogen,pyridine-like and pyrrole-like nitrogen,pyrrolic nitrogen and graphitic nitrogen of ZIF-8-800 crystal are 398.10,399.40, 400.55, and 402.47 eV, respectively. While the binding energy of pyridinic nitrogen, pyridine-like and pyrrole-like nitrogen and pyrrolic nitrogen shifted to low field, decreasing 0.06,0.64, and 0.18 eV, respectively in the spectrum of Sn-ZIF-8-800-20%. It indicates the formation of chemical nitrogen through the interaction between ZIF-8-800 and Sn, which increases the electron cloud density on N (the electronegativity of Sn is smaller)and could effectively enhance conductivity of porous material and regulate the electron transfer from catalyst to CO2. The lone electron pair in the nitrogen atom as a carrier can also improve the ability of the porous carbon material to ‘‘a(chǎn)ttract” metal ions,which contributes to the dispersion of Sn on ZIF-8-800 to provide more catalytic activity site. The carbonized ZIF-8 has the ability to adsorb CO2, especially the basic nitrogen functional group contained in the nitrogen doping can effectively increase the adsorption capacity of acid gas [29,30].Besides, the pyrrolic nitrogen in carbon materials could enhance the adsorption capacity of CO2through quadrupole interaction and van der Waals interaction[28], therefore is helpful to enhance the catalytic activity of CO2.

Fig. 7. Nyquist plots for Sn and Sn-ZIF-8-T-20% electrodes in CO2-saturated 5 mol · L-1 KHCO3 electrolyte. The inset is Randles equivalent circuit (Rs, Rct, Cdl, and Zw stand for the internal resistance including electrolyte,membrane and electrodes,charge transfer resistance, double-layer capacitance and finite length Warburg impedance element, respectively).

On the other hand,the increase in the binding energy of graphitic nitrogen was due to the introduction of Sn, which makes the ZIF-8-800 crystal structure partially disordered due to the reconstruction, and the appearance of high energy peaks is usually attributed to the nitrogen oxide [31]. The results have shown that pyridinic nitrogen has high catalytic activity for oxygen reduction.For example,the pyridine-type nitrogen vacancy defect synergistic[32], the pyridine-type nitrogen creates the Lewis base site [33]and other theories.The adsorption of reactants on the catalyst surface is a prerequisite for the progress of the reaction.Therefore,the interaction of Sn and nitrogen-doped porous carbon materials

ZIF-8-800 doping not only affects the grain size and BET surface area,but also changes the electron binding energy of the Sn 3d,as shown in Fig.6(b).It can be seen that the doping of ZIF-8-800 will shift the Sn 3d binding energy to the low field,Sn 3d 3/2 and Sn 3d 5/2 decreased by 0.25 eV and 0.21 eV, respectively, indicates that the electron density of the Sn 3d orbital atom increases.The reason is that ZIF-8-800 nano microcrystals modify the surface of Sn grains. Due to the difference in electronegativity of the elements at the location where the crystal planes meet, the electron shift between the atoms of different elements is driven. The increase of electron density of the Sn 3d orbital helps to improve its electrical conductivity, because Sn is an n-type semiconductor and its electrical conductivity comes from quasi-free electrons. The increase in the conductivity of the Sn-ZIF-8-800-20% catalyst will effectively regulate the electron transport and promote the electron transport from the electrode to the reactant during the catalytic reaction, thereby improving the catalytic activity.

3.2. Charge and mass transfer resistance of Sn doped with ZIF-8-T

The doping of ZIF-8-T in the catalyst Sn also helps to improve the mass transfer properties of CO2hydrogenation,as investigated through electrochemical impedance spectroscopy(EIS)and shown in Fig. 7 and Table 1. The intersection of the semicircle and the X-axis represents the internal resistance of the electrolyte and the electrode, that is, the solution ohmic resistanceRs, which has little change. The radius of the semicircle represents the charge transfer resistanceRct, Sn-ZIF-8-800-20% catalyst also exhibits the lowest interface charge transfer resistance (0.94 Ω), which makes it easier for electrons to transfer from the electrode to the adsorbed CO2molecules to form the reduction intermediate carboxylate *CO2-(* denotes the surface-coordinated state of the ligand, and is the rate determining step of CO2RR). The straight lines with different slopes in low frequency region represent Warburg impedance, which corresponds to the mass transfer and diffusion process of the active material in the electrolyte and shows the lowest value in the Sn-ZIF-8-800-20% catalyst because it has the best crystal morphology.

Table 1 Impedance parameters of commercial Sn powder, and Sn doped Sn-ZIF-8-T-20% obtained according to equivalent circuit fitting in electrochemical hydrogen pump reactor

Table 2 Comparison of electrocatalytic properties of the catalyst with typical Sn-based catalysts reported recently

3.3. Effect of nitrogen doped porous carbon materials on electrochemical performance of CO2 hydrogenation

Sn-ZIF-8-800 could significantly improve electrocatalytic activity of CO2hydrogenation, as shown in the linear voltammetry curve (LSV) in Fig. 8. The current density difference between the CO2saturated and N2saturation conditions corresponds to the catalytic activity of the CO2hydrogenation, that is, the CO2reduction capacity.Only using carbonized ZIF-8-800 as catalyst can improve the current density of the system,however,the current density has no obvious difference in the atmosphere of N2and CO2,which suggests hydrogen evolution dominant on ZIF-8-800. We also analyzed the product as ZIF-8-800 was used as catalyst, HCOOH could not be detected by NMR spectra and very little CO was detected by GS.Excessive doping amount of ZIF-8-800 would affect the dispersion of Sn,which is not conducive to the progress of CO2hydrogenation reaction but increases hydrogen evolution. As a result, doping 10% (mass) and 20% (mass) ZIF-8-800 shows better abilities of hydrogen evolution inhibition and CO2reduction as compared with commercial Sn catalyst. Different doping amount of ZIF-8-800 also affected the current density, Sn-ZIF-8-800-20%exhibits the best electrocatalytic activity. The current density difference is 16.8% higher than that of commercial Sn catalyst (64.6vs.55.3 mA·cm-2). It shows that Sn-ZIF-8-800-20% has a certain stability during electrolysis process as shown in Fig. 8(b). After 30 h of continuous potentiostatic test, Faradaic efficiency of all products remained stable, and the total FE value keeps at about 70%.

Fig.8. Linear sweep voltammetry and stability test.(a)LSV curves of Sn-ZIF-8-800-10%, Sn-ZIF-8-800-20%, Sn-ZIF-8-800-30% and ZIF-8-800 after being electrochemically reduced at different cathode potential in a CO2-saturated (solid line) and N2-saturated (dashed line) 0.5 mol · L-1 KHCO3 aqueous solution, the catalyst load is 2 mg·cm-2, the scan rate is 0.05 V·s-1. (b) Sn-ZIF-8-800-20% stability test at-1.34 V vs. RHE for 30-hour continuous potentiostatic test.

Fig. 9. Electrochemical hydrogenation performance. (a) total Faraday efficiency of CO2 electrocatalytic hydrogenation, Faradaic efficiency and formation rate of (b) HCOOH,(c)CO,(d)H2 during 30 min reaction in 0.5 mol·L-1 KHCO3 electrolyte for Sn(dashed line)and Sn-ZIF-8-800-20%(solid line)(Formation rate:black line,Faradaic efficiency:blue line).

Doping of ZIF-8-800 in commercial Sn also significantly improves the reaction rate and Faraday efficiency of CO2electrocatalytic hydrogenation. As shown in Fig. 9, at -1.14 Vvs.RHE cathode voltage, total Faraday efficiency of hydrogenated product HCOOH and CO produced by doping 20%(mass)ZIF-8-800 reaches 68.9%,but the commercialized Sn is only 45.0%at the same potential.Formation rates of HCOOH and CO increase by 68.7%(141.9vs.84.1 μmol·h-1·cm-2) and 109.3% (96.5vs.46.1 μmol·h-1·cm-2),respectively. Faraday efficiency of HCOOH and CO increased by 43.6% (41.8%vs.29.1%) and 70.4% (27.1%vs.15.9%) respectively,indicating that CO is more easily desorbed from Sn-ZIF-8-800-20% catalyst surface rather than participating in further reactions.It should be noted that the synergistic effect between ZIF-8-800 and Sn as evidenced through XPS in Fig.6 reduces the onset potential of the CO2electroreduction.As shown in Fig.9(b),the reaction onset potential of HCOOH decreased from-0.74 Vvs.RHE reduced to less than -0.54 Vvs.RHE. The improvement of hydrogenation product selectivity is closely related to the micro-mesoporous structure, high specific surface area and excellent mass transfer performance of the nitrogen-doped porous carbon material ZIF-8-800. In Table 2, we compare the electrocatalytic performance of the multistage pore structure catalyst prepared in this paper with the recently reported Sn based catalyst. Key parameters such as Faraday efficiency of the target product are listed in the table.Through comparison, it can be found that the catalysts prepared in this paper show better overall performance and maintain high reaction selectivity at high potential. The Sn-ZIF-8-800 catalyst in this article has better performance than some Sn-based catalysts reported in the literature (for example, the CO2RR Faraday efficiency was 47% at -2.00 Vvs.SCE used Sn powder as catalyst[35], 40.7% at -0.70 Vvs.RHE when Sn/SnOx used as catalyst[36],27.2%at-1.70 Vvs.SCE when SnO2/MWCNT used as catalyst[43], 46% at -1.30 Vvs.Ag/AgCl when SnO2/N-MWCNTs used as catalyst [31]).

4. Conclusions

This work has successfully synthesized ZIF-8 with Zn(NO3)2-·6H2O and 2-methylimidazole at room temperature,and the effects of different carbonization temperatures on the surface morphology and catalytic performance of ZIF-8-T crystals after carbonization were investigated. SEM and XRD show that the structure of ZIF-8-T crystal is destroyed with the increase of carbonization temperature. The higher the temperature, the more obvious the collapse of the carbide surface.When the temperature is 800°C,the surface shape of the sample is the most complete, and C and N are uniformly distributed, no agglomeration occurs, and the specific surface area of ZIF-8-800 is larger. The results of EIS and LSV show that ZIF-8-800 electrocatalyst with carbonization temperature of 800 °C and Sn doped with 20% (mass) ZIF-8-800 electrocatalyst has the best electrochemical activity.Compared with the commercial Sn powder, the CO2catalytic hydrogenation reaction shows that Sn- ZIF-8-800-20% electrocatalyst can effectively improve the reaction performance. Reaction rate and Faraday efficiency of the hydrogenation product HCOOH and CO are both improved. At-1.14 Vvs.RHE, the Faraday efficiency of HCOOH increased from 29.1% to 41.8%, and the Faraday efficiency of CO increased from 15.9% to 27.1%, and the reaction onset potential decreased from-0.74 Vvs.RHE to-0.54 Vvs.RHE.This is due to the high specific surface area and CO2adsorption of ZIF-8-800, as well as effective regulation and catalytic effect of nitrogen-doped porous carbon material on electron transport. In conclusion, ZIF-8-800 nitrogendoped porous material can promote the adsorption and mass transfer of CO2, and pyridinic nitrogen can effectively increase the electrochemical performance of electrochemical reduction of CO2catalytic hydrogenation.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Joint Fund U1663223 and 21776034), Science Fund for Creative Research Groups of the National Natural Science Foundation of China (22021005), the National Key Research and Development Program of China(2016YFB0101203), Educational Department of Liaoning Province of China(LT2015007),Fundamental Research Funds for the Central Universities (DUT16TD19) and the Changjiang Scholar Program(T2012049).