Research Article pubs.acs.org/journal/ascecg
Effect of Cu(I)−N Active Sites on the N2 Photofixation Ability over Flowerlike Copper-Doped g‑C3N4 Prepared via a Novel Molten SaltAssisted Microwave Process: The Experimental and Density Functional Theory Simulation Analysis Shaozheng Hu,* Xiaoyu Qu, Jin Bai, Ping Li, Qiang Li, Fei Wang, and Lijuan Song College of Chemistry, Chemical Engineering, and Environmental Engineering, Liaoning Shihua University, Dandong Road 1#, Wanghua District, Fushun 113001, China ABSTRACT: Flowerlike copper-doped g-C3N4 is synthesized via a novel molten salt-assisted microwave process in this work. X-ray diffraction, N2 adsorption, UV−vis spectroscopy, scanning electron microscopy, photoluminescence, temperature-programmed desorption, X-ray photoelectron spectroscopy, and electrochemical impedance spectra were used to characterize the prepared catalysts. The results show that Cu+ is not present as oxide but inserts at the interstitial position through the coordinative Cu(I)−N bonds. These Cu(I)−N active sites can act as chemical adsorption sites to activate N2 molecules. Moreover, as an “electron transfer bridge”, Cu(I)− N active sites promote electron transfer from the catalyst to the adsorbed N2 molecules. The as-prepared copper-doped g-C3N4 displays a much higher NH4+ generation rate than neat g-C3N4 prepared by calcination, as well as excellent catalytic and structural stability. Density functional theory simulations prove that Cu(I)−N active sites can adsorb the N2 molecule with high adsorption energy and elongate the NN bond. Charge density difference result confirms the electrons transfer from the Cu+ doping sites to the N2 molecule. Density of states results indicate that the σg2p orbital in nitrogen atom is delocalized significantly when N2 is adsorbed on Cu+ doping sites; also, the πg*2p orbital is transferred to the vicinity of the Fermi level. These make the nitrogen molecules more efficient to activate. KEYWORDS: g-C3N4, Nitrogen photofixation, Molten salt-assisted microwave process, Cu(I)−N active sites, Density functional theory simulation
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Schrauzer et al. for the first time found that Fe-doped TiO2 powder can photocatalytically reduce nitrogen to form ammonia.1 Since then, many photocatalysts, such as precious metals supported on TiO2, Fe2Ti2O7, graphite-phase carbon nitride, bismuth oxyhalide, and polymetallic sulfide, have been reported successively.2−6 However, photocatalytic nitrogen fixation is not a thermodynamically spontaneous reaction. The NN bond energy is as high as 941 kJ·moL−1 with a first ionization potential of 15.58 eV. Thus, N2 photofixation cannot be achieved under atmospheric pressure and room temperature, unlike with nitrogenase. Graphite phase carbon nitride (g-C3N4), as the darling of the catalytic community in recent years, has special physical and chemical properties, excellent chemical stability, and adjustable electronic structure.7−12 However, the g-C3N4 prepared by ordinary high-temperature polycondensation methods has a large grain size, resulting in a small specific surface area of only ∼10 m2·g−1, which limits its application in the field of catalysis
INTRODUCTION NH3, as a hydrogen-rich material, has received widespread attention because of its high energy density (hydrogen density of 17.6 wt %) and easy storage and transportation. Nitrogen, as a raw material for synthetic proteins, is also an essential element for human and other biological growth. Although nitrogen accounts for 78% of the atmosphere, the vast majority of organisms cannot directly use nitrogen from the air. Therefore, nitrogen fixation is the most important chemical reaction in nature after photosynthesis. At present, the use of the Haber method as an industrial nitrogen fixation process has reached an annual level of 100 million tons. This method requires high temperature and pressure as well as the presence of hydrogen. Not only are the raw material costs and energy consumption high for this process, there is a certain risk. Therefore, a green, clean, and low energy consumption process to replace the Haber method could both solve the problem of hydrogen energy storage and also alleviate the high energy consumption and serious environmental pollution problems of the Haber method. With the continuous development of heterogeneous photocatalytic technology, photocatalytic nitrogen fixation technology has been widely studied by researchers. In 1977, © 2017 American Chemical Society
Received: April 10, 2017 Revised: May 23, 2017 Published: July 2, 2017 6863
DOI: 10.1021/acssuschemeng.7b01089 ACS Sustainable Chem. Eng. 2017, 5, 6863−6872
ACS Sustainable Chemistry & Engineering
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and optoelectronic materials.13 Therefore, shape control preparation of g-C3N4 catalyst with large specific surface area has become the focus and difficulty of this research field. Microwave is a heating method that is completely different from other conventional methods. When the microwave energy is absorbed by the raw material, the molecules are arranged in an orderly fashion in the electromagnetic field of the microwave. Then, the high frequency reciprocating motion causes the frequent collisions between molecules, leading to the generation of frictional heat. Under this method, the raw material can be rapidly heated without the presence of a temperature gradient.14 Yu et al. successfully prepared a g-C3N4 catalyst with high specific surface area by the microwave method in a short time and showed good catalytic performance.15 This method also has the advantages of low energy consumption, low harmful gas emission, high product yield, and so on. In recent years, the molten salt method has been widely used in the field of material synthesis. This method accelerates the diffusion of reactant ions, controls the crystal growth of the product, and can be easily separated from the product by washing.16−18 Not only that, the molten salt method minimizes the interfacial energy between the product and the molten salt, resulting in a special product morphology. However, the conventional resistance hot-melt salt method exhibits the problems of long running time, high energy consumption, and low product yield. Thus, it is proposed that the combination of the advantages of the microwave and molten salt process will be an ideal method for preparing a g-C3N4 catalyst with high catalytic performance. As mentioned above, because of the high NN bond energy, the key to photocatalytic nitrogen fixation is the activation of N2 molecules. Li, Dong, and Hu et al. have successively found that oxygen vacancy, nitrogen vacancy, and sulfur vacancy in BiOBr, g-C3N4, and polymetallic sulfides can be used as reactivity centers to active N2 molecules.4−6 On the one hand, vacancy can chemical adsorb the N2 molecules to active the NN bond. On the other hand, vacancy can capture photoelectrons and promote the photoelectron transfer from the catalyst to the N2 molecules, significantly improving the nitrogen fixation performance.4−6 Thus, we imagine that the metal cation doping, as another surface defect, may also serve as an active site to promote the N2 phorofixation performance. Based on the above assumptions, in this work, a Cu+ doped flowerlike g-C3N4 with excellent photocatalytic nitrogen fixation activity under visible light was prepared via a novel molten salt-assisted microwave process. Molten salt wrapped the raw materials during the catalyst preparation process, which prevents the copper from contacting the oxygen. Thus, as the active site, Cu+ exists in the formation of Cu(I)−N bond rather than nonactive copper oxide. On the one hand, Cu(I)−N active sites can effectively capture photoelectrons and rapid transfer these electrons to the adsorbed N2 molecules based on the electron back-donation phenomenon.19−21 On the other hand, the molten salt-assisted microwave method changed the morphology of as-prepared g-C3N4 catalyst from the layered structure to nanoparticles and self-assembled into a flowerlike morphology with a large specific surface area. The density functional theory (DFT) calculation results confirm the proposed process of N2 molecules activation by photoelectron.
Research Article
EXPERIMENTAL SECTION
Preparation and Characterization. A 6 g portion of dicyandiamide and a desired amount of copper phthalocyanine (molar ratio Cu/dicyandiamide = 0.01) were added into 20 mL of ethanol. The solution was heated at 60 °C for evaporating ethanol. The solid was transferred to a crucible. KCl-LiCl (1:1 weight ratio), as eutectic salts, were added into the crucible (weight ratio eutectic salts/ dicyandiamide = 50:1). This crucible was then put into another alumina crucible (200 mL), buried with the CuO powder for absorbing microwave energy, and treated by microwave for 30 min in a normal microwave oven (G70D20CN1P-D2, Galanz). The obtained catalyst was denoted as MS-Cu-CN. For comparison, MW-CN was prepared following the same procedure as in the synthesis of MS-CuCN but in the absence of eutectic salts and copper phthalocyanine. Bulk g-C3N4 was prepared by heating dicyandiamide at 550 °C for 4 h at the rate of 5 °C/min. The product was denoted as GCN. In order to confirm the reaction active site, MS-Cu-CN was annealed at 450 °C in O2 atmosphere (90% Ar and 10% O2) for 1, 2, and 3 h. The obtained sample was denoted as MS-Cu-CN(1h), MS-Cu-CN(2h), and MS-CuCN(3h), respectively. CuO/GCN composite, with the same Cu concentration as MS-Cu-CN, was also prepared according to previous work by using CuO instead of Fe2O3.22 The XRD patterns were carried out on a Rigaku D/max-2400 instrument using Cu Kα radiation (λ = 1.54 Å). UV−vis spectroscopy was performed on a UV−vis spectrophotometer (JASCO V-550) using BaSO4 as the reflectance sample. A scanning electron microscope (SEM, JSM 5600LV, JEOL Ltd.) was used to analyze the morphologies of catalysts. Nitrogen adsorption was measured at −196 °C on a Micromeritics 2010 analyzer. The BET surface area (SBET) was calculated based on the adsorption isotherm. Fourier transform infrared spectra (FT-IR) were obtained on a Nicolet 20DXB FT-IR spectrometer. Inductively couple plasma (ICP) analysis was performed on a PerkinElmer Optima 3300DV apparatus. The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Escalab 250 XPS system with Al Kα radiation as the excitation source. Temperature-programmed desorption (TPD) studies were performed using a CHEMBET-3000 (Quantachrome, U.S.A.) instrument. The photoluminescence (PL) spectra were measured at room temperature with a fluorospectrophotometer (FP6300) using a Xe lamp as the excitation source. Electrochemical impedance spectra (EIS) was performed via an EIS spectrometer (ECLab SP-150, BioLogic Science Instruments) in a three-electrode cell by applying 10 mV alternative signal versus the reference electrode (SCE) over the frequency range of 1 MHz to 100 mHz. All calculations reported in this work were carried out in the framework of DFT using the Cambridge Serial Total Energy Package (CASTEP) plane-wave code in Materials Studio 5.5 software, Accelyres. The exchange and correlation interactions were modeled using the Perdew−Wang 91 functional within the generalized gradient approximation (GGA). The wave functions of the valence electrons were expanded using a plane-wave basis set within a specified cutoff energy of 400 eV. Electronion interactions were described by the ultrasoft pseudopotential. The substrate is modeled by one layer of gC3N4 separated by a vacuum layer of 12 Å. All the atoms in the layer and the N2 molecule are allowed to relax. The Brillouin zones of the supercells were sampled by the Gamma points. Based on the structures of g-C3N4, the g-C3N4 surface with Cu+ doping was modeled to study the N2 adsorption properties. Photocatalytic Reaction. The nitrogen photofixation performance of prepared catalysts was carried out according to previous work.23 The experiments were performed in a double-walled quartz reactor in air. A 0.2 g portion of catalyst was added to a 500 mL deionized water. Ethanol (0.789 g·L−1) was added as a hole scavenger. The suspension was dispersed using an ultrasonicator for 10 min. A 250 W high-pressure sodium lamp with main emission in the range of 400−800 nm was used as the light source. The UV light portion of the sodium lamp was filtered by NaNO2 solution (0.5 M). A 5 mL portion of the suspension was collected at given time intervals and immediately centrifuged to separate the liquid samples from the 6864
DOI: 10.1021/acssuschemeng.7b01089 ACS Sustainable Chem. Eng. 2017, 5, 6863−6872
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. NH4+ production ability over as-prepared catalysts (a), NH4+ production ability of MS-Cu-CN using AgNO3 as the electron scavenger (b) or in aprotic solvents DMF and DMSO (c), the pH value change of MS-Cu-CN suspension during the nitrogen photofixation process (d), the mass spectra of the indophenol prepared from different atmosphere (e), and the N2 photofixation ability and stability of V-g-C3N4, BiOBr, ZnSnCdS, and Cu(2)-SCN (f). solid catalyst. The concentration of NH4+ was measured using the Nessler’s reagent spectrophotometry method (JB7478-87) with a UV2450 spectrophotometer (Shimadzu, Japan).18,19
Table 1. Influence of Cu/Dicyandiamide Molar Ratio on the Nitrogen Photofixation Performance
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RESULTS AND DISCUSSION Figure 1a displays the N2 photofixation ability over as-prepared catalysts. The control experiment indicates that the NH4+ concentrations are very low (
