Nitrogen-Doped Carbon Cages Encapsulating CuZn Alloy for

Jun 20, 2019 - Elemental distribution of N species in the metal-free PAN is similar to the C–N shell of ... (52) As for the spectrum in the O 1s reg...
1 downloads 0 Views 3MB Size
Download PDF
Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

www.acsami.org

Nitrogen-Doped Carbon Cages Encapsulating CuZn Alloy for Enhanced CO2 Reduction Xiaosong Hu,† Chaoyue Zhao,† Xin Hu,† Qingxin Guan,*,† Yanlin Wang,† and Wei Li*,†,‡ †

State Key Laboratory of Elemento-Organic Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China

Downloaded via UNIV OF SOUTHERN INDIANA on July 17, 2019 at 06:30:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: CuZn alloy, regarded as the active sites, shows excellent catalytic activity for the reverse water gas shift reaction, whereas the incorporation of N atoms, especially pyridinic N, can greatly improve its catalytic properties because of the strong promotion capacity for adsorption and activation of CO2 molecules. Herein, the synthesis strategy involving Cu-doped Zn-based metal−organic frameworks is utilized to prepare CuZn alloy coated in an N-doped carbon shell. The excellent catalytic ability for CO2 transformation originates from the synergistic catalytic effect between CuZn alloy and pyridinic N. The strong adsorption and activation capacity for CO2 of pyridinic N is ascribed to the lone pair of electrons on the N atom and the high electron density in its vicinity.

KEYWORDS: alloy, core−shell, CO2, metal−organic frameworks, pyridinic N

1. INTRODUCTION Continued increases in CO2 emissions are the primary cause of global warming.1,2 Considering that massive emissions of CO2 lead not only to serious environmental problems but also to significant economic issues,3 tremendous efforts, such as CO2 capture, storage, and transformation, have been devoted to reducing the environmental impacts of CO2.2,4 In particular, the hydrogenation reduction of CO2 is regarded as the effective way to consume excessive CO2 in the atmosphere, and it also shows great economic benefits.2,5,6 In this respect, CO2 can be a sustainable reductive transformation to value-added carbon products, such as CO, methanol, methane, and other carbonbased fuels.7−11 In general, the high stability of the CO2 molecule in kinetics and thermodynamics hindered its applications.12 Among the various CO2 reduction products, there are only two electrons to be transferred for the generation of CO compared with methanol (six electrons) or methane (eight electrons).6 In addition, reduction of CO2 to CO is considered as an attractive strategy for producing syngas, which can be used to synthesize many useful chemicals via the Fischer−Tropsch reaction.13,14 Thus, hydrogenation of CO2 to CO via the reverse water gas shift (RWGS) reaction is regarded as a prospective way for CO2 chemical transformation.15 Copper-based catalysts are widely investigated in the evaluation of CO2 hydrogenation because of their excellent catalytic activity and product selectivity, especially in the synthesis of methanol and CO.15 Impressively, a Cu/ZnO catalyst has been developed for CO2 hydrogenation to CO and © XXXX American Chemical Society

methanol under high-temperature and high-pressure conditions because of the excellent promotion of ZnO. In general, Cu is considered as the main active component, whereas the reason why ZnO plays a key role is still controversial. Recently, a CuZn bimetallic alloy was found in the interface of Cu and ZnO, which is connected with the high activity of the catalyst in CO2 hydrogenation.1,16,17 In most instances, bimetallic alloy catalysts display superior catalytic activity when compared with the individual metals. The catalytic activities will be promoted because the constitution of intrinsic polarity comes from the bonding of different metals.18 In the CuZn catalyst system, the electronic structure of Cu will be changed resulted from the decoration of Zn atoms on the interface of Cu particles, whereas these Zn atoms come from the reduction of ZnO.16 It means that Cu steps decorated with some Zn atoms form the active sites, which contain the abundant defects.19 Studies involving CuZn alloy have caused great interest in view of its abundance, nontoxicity, and excellent catalytic effects on CO2 reduction.19−21 However, investigations on the application of CuZn alloy in the RWGS reaction are absent. It is worth exploring the catalyst properties and catalytic mechanism of the CuZn alloy in CO2 hydrogenation and transformation to CO. The adsorption ability of reactant molecules on the surface of a catalyst is an important indicator for further catalytic Received: February 25, 2019 Accepted: June 20, 2019 Published: June 20, 2019 A

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

temperature oven (180 °C) and maintained for 3−18 h. After the synthesis reaction is completed, the hydrothermal kettle is then cooled to room temperature. A Cu-doped ZIF-8 precursor can be obtained by washing and drying. It is necessary that more than 6 h of drying was implemented in a constant temperature oven at 80 °C. The product was marked as Cu@ZIF-8. 2.3. Synthesis of CuZn Alloy@C−N. First, the above-synthesized Cu NPs@ZIF-8 MOFs were placed in a tube furnace and warmed up to 300 °C in N2 atmosphere for removing the residual 2methylimidazole and replace the air in the sample. Then, the temperature of the tube furnace was further warmed up to 500 °C after 1 h. CuZn alloy@C−N can be obtained after 4 h of calcination at 500 °C. 2.4. Synthesis of CuZn Alloy. The mixed oxide CuZnOx was placed in a tube furnace and warmed up to 500 °C in N2 atmosphere. Then, the reduction for another 4 h is necessary at the same temperature under H2 flow (60 mL/min) after continuous calcination in N2 for 1 h. Finally, the CuZn alloy was obtained after cooling to room temperature with N2 flow. 2.5. Polyacrylonitrile Pretreatment. At the first stage, polyacrylonitrile (PAN) was heated to 270 °C in air and the temperature was maintained for 2 h. Then, the temperature was further increased to 500 °C under N2 flow. Finally, the pretreated PAN sample was obtained after continuing 1 h calcination at 500 °C. 2.6. Characterization. The measurement of Fourier transform infrared (FTIR) spectra of the catalysts was performed by a Thermo Scientific Nicolet iS5 spectrometer. Thermogravimetric analysis (TGA, TG209) was performed under N2 flow. X-ray absorption fine structure (XAFS) spectra were tested in Beijing Synchrotron Radiation Facility (1W1B station, BSRF). The voltage and maximum current used in the test were 2.5 GeV and 250 mA, respectively. The detailed information involving other characterization techniques in this work is consistent with the description in our previous report.32 2.7. Catalytic Evaluation. Hydrogenation of CO2 to CO by the RWGS reaction was implemented in a stainless steel reactor (fixedbed). The catalyst (100 mg) was evenly mixed with 1 mL quartz sand, and then the mixture was placed in the suitable location of the tube reactor. The temperature error is controlled within ±1 °C. The catalytic activity was tested at 350−500 °C under the constant gas flow of H2 and CO2. The catalyst should be reduced for 4 h at 500 °C in the H2 flow (60 mL·min−1) when CuO−ZnO was used to the RWGS reaction. Furthermore, the reaction system was pressurized to target pressure by the influx of reaction gases. The reactant and products were analyzed by the gas chromatograph online under the H2 flow. An area normalization method was available for calculating CO2 conversion and product selectivity. The investigation of kinetics was performed on the same conditions except for the different temperatures.

reactions. The capture and enrichment of reactants should be considered when focusing on designing active sites for the RWGS reaction, which contributes to the catalytic conversion of CO2.22 Recently, researchers have paid attention to investigations of pyridinic nitrogen and established a close link between pyridinic nitrogen and CO2 reduction, especially in electrochemical reduction.23−29 Density functional theory (DFT) calculations have provided a theoretical basis for the strong adsorption of CO2 and the intermediate species formation of *COOH.24 The pyridinic nitrogen defects exhibit a high activity for catalyzing the hydrogenation reduction of CO2, which is ascribed to the decrease of free-energy barrier that led to the formation of adsorbed *COOH and further promote the generation of CO.25 In order to get the target product, CO must be easily desorbed from the surface of the catalyst to prevent further hydrogenation. Thus, the low desorption energy of CO* in pyridinic nitrogen further promotes the formation of CO.25 Therefore, synergistically combining CuZn alloy and pyridinic nitrogen in the formation of a composite catalyst is a logically prospective strategy for developing a novel RWGS catalyst with excellent catalytic performance. Herein, we successfully synthesized a CuZn nanoalloy encapsulated by N-doped carbon shells, and the catalyst exhibited the high catalytic performances for thermal catalysis in the RWGS reaction. The present strategy involved a solventfree solid-phase synthesis method to produce the Cu@ZIF-8 precursor, followed by one-step high-temperature pyrolysis to obtain the target composite material (CuZn@C−N). The catalyst exhibited a great potential to catalyze CO2 hydrogenation to CO. Interestingly, it was found that pyridinic nitrogen dominated the N-containing species, and the catalytic behavior in RWGS was significantly enhanced because of the promotion by pyridinic nitrogen. This strategy is effective and facile for the preparation of core−shell composite catalysts, where the metallic alloy core was encapsulated by a uniform carbon shell functionalized by the abundant pyridinic nitrogen. The unique core−shell structure will hinder the inactivation of the alloy core and simultaneously enhance the synergistic effects between core and shell in catalysis.30,31

2. EXPERIMENTAL SECTION 2.1. Chemicals. The specifications of the reagents used in the experiment are of analytical purity. 2.2. Synthesis of Cu NPs@ZIF-8 MOFs. For synthesizing the sample of Cu NPs@ZIF-8 MOFs, the aqueous solution containing copper(II) nitrate and zinc nitrate was first prepared, where the molar ratio between Cu and Zn is equal. The aqueous solution of Na2CO3 was used as the precipitant, and both the concentrations of CO32− and metal ions (Cu2+ and Zn2+) were 0.25 M. The two solutions were then mixed at uniform droplet velocities. The temperature needs to be kept at 65 °C, whereas the pH between 7 and 8 is necessary. After the solutions were dripped completely, the mixed solution needs to be stirred for 1 h at this temperature. Subsequently, the suspension was stopped stirring and placed for 2 h to cool it down. A solid sample can be obtained after filtration, washing, and drying. More than 4 h of drying was implemented in a constant temperature oven at 100 °C. Finally, the mixed oxide CuZnOx was obtained by calcining the sample for more than 4 h in a tube furnace at 400 °C in N2 atmosphere. The mixed oxide CuZnOx was evenly mixed with 2-methylimidazole by using a solid-phase grinding. The molar ratios between Zn and 2-methylimidazole are 1:0.5, 1:1, 1:3, and 1:5. After 1 h of grinding, the sample was transferred to a hydrothermal kettle with a Teflon liner. Then, the hydrothermal kettle was put in the constant

3. RESULTS AND DISCUSSION The scheme of preparing the CuZn@C−N catalyst is presented in Figure 1a. The formation of Cu@ZIF-8 by etching the mixed metal oxides (CuO−ZnO) using 2methylimidazole not only caused the metal−organic frameworks’ (MOFs) structure to be coated on the outside of the metal but is also beneficial to the dispersion of the metal core. The MOF-derived synthesis is an ideal way to fabricate the highly nitrogen-doped carbon cages encapsulating the CuZn alloy. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images can reveal the morphology of Cu@ZIF-8 and CuZn@C−N as well as the dispersion of the metal core clearly and intuitively. The increase in the molar ratio between the ligand (2-methylimidazole) and ZnO causes a significant reduction in surface particle size of metal oxides because of the etching action of the ligand (Figure 1(b-1−e1)). On the basis of this etching action, a hydrophobic angle of about 128° confirms that a hydrophobic shell is coated on the B

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

under the further high-temperature calcination after the generation of [email protected],35,36 The MOF (ZIF-8) shell is also pyrolyzed into a nitrogen-containing carbon skeleton in this stage. The crystal structure of samples was characterized by X-ray diffraction (XRD). There are no diffraction peaks of CuO present in the pattern of Cu@ZIF-8 except for those peaks that well match to metal Cu (JCPDS card no. 04-0836) (Figure S4, Supporting Information).37 The disappearance of CuO diffraction peaks (JCPDS card no. 48-1548) is mainly due to the coordination between 2-methylimidazole and CuO, whereas the reduction that occurs during solid-phase etching is the main reason for the existence of a metallic copper phase.38 As the molar ratio increases between the ligand (2methylimidazole) and ZnO, the diffraction peaks of Cu gradually decrease and the related diffraction peaks involving ZIF-8 are also becoming more and more obvious, indicating that increasing the content of 2-methylimidazole will promote the generation of ZIF-8.37,39 In particular, ZnO will be completely etched into ZIF-8 when the molar ratio exceeds 3:1. Furthermore, the dispersion of Cu is also promoted because of the restriction of the MOF skeleton.39 The TGA measurement of the Cu@ZIF-8 sample was conducted to understand the process about decomposition and carbonization of Cu@ZIF-8 into CuZn@C−N with the increase of temperature (Figure S5, Supporting Information). The weight loss of the Cu@ZIF-8 sample mainly occurs above 450 °C, which indicates that the skeleton of MOFs can be converted into a carbon skeleton containing N under the experimental condition (500 °C). As observed in Figure 2a, hightemperature calcination causes complete destruction of the ZIF-8 frame structure. Only three diffraction peaks are found in the CuZn@C−N samples, and these peaks are all shifted to a lower angle relative to the characteristic diffraction peaks of Cu (JCPDS card no. 04-0836). Thus, it is likely to be a reasonable explanation that during the calcination process, several Zn atoms have embedded into the Cu crystal, causing crystal lattice expansion as well as the crystal lattice spacing increases, which leads to the shift of the diffraction peaks to a smaller angle.17,40,41 The XRD patterns exhibit the peaks’ shift to the lower angle side when the molar ratio of 2methylimidazole is decreased (Figure 2a), which suggests the increase of crystal lattice spacing. However, as the etching time increases, the angles of the peak shifts are almost the same (Figure S6). It is confirmed that the etching time makes a smaller contribution to the change in the crystal structure of CuZn than the molar ratio between ligand and ZnO. The XAFS measurement was performed to detect the structure of the CuZn@C−N samples at atomic and electronic levels. The Cu K-edge extended XAFS (EXAFS) spectra, which are presented as a Fourier transform, are shown in Figure 2b. The main peak of the Cu foil appeared at 2.14 Å, which is assigned to single scattering of the nearest-neighbor Cu−Cu.17,42,43 The Cu−Cu peak of the CuZn@C−N samples shifts to the higher coordination slightly as well as the signal intensity also decreases significantly when compared with the Cu foil, indicating that Zn atoms interact with Cu atoms.19,30,43 Compared with the high molar ratio (5:1), the low molar ratio (1:1) between the ligand and ZnO leads to a greater positive shift (2.26 Å). The Cu K-edge X-ray absorption nearedge structure (XANES) measurement shows that the intensity of the white line peaks in the CuZn@C−N samples is weaker than that of the Cu foil, and the position of the Cu absorption

Figure 1. Schematic illustration of the synthetic route and characterization of the Cu-doped ZIF-8 precursor and CuZn@C−N sample. (a) Schematic illustration of the synthetic route for CuZn alloy encapsulated in the N-doped carbon shell layers. SEM images of the Cu-doped ZIF-8 precursor (b-1−e-1) and TEM images of the CuZn@C−N sample (b-2−e-2,b-3−e-3) with different molar ratio between the ligand and ZnO (b) 0.5:1, (c) 1:1, (d) 3:1, (e) 5:1 (the etching time of all the samples is 18 h). (f) Hydrophobic angle measurement of the Cu-doped ZIF-8 precursor (5:1, 18 h). (g) EDXmapping for the corresponding elemental distribution of Cu, Zn, C and N recorded on a single CuZn@C−N nanoparticle (g-1−g-5).

outside of the metal oxide core (Figure 1f). It is well known that hydrophobicity is a characteristic of ZIF-8.33,34 During the calcination process, the metal and metal oxides will be formed by sintering, but the presence of the coated carbon skeleton will prevent the occurrence of sintering. Thus, the raise of the molar ratio between the ligand and ZnO enhances the dispersion of the metal core (Figure 1(b-2−e-2)). In addition, the thickness of the shell relative to the metal core also increases as this ratio increases (Figure 1(b-3−e-3)). The surface situation of metal oxides, the thickness of the shell, and the dispersion degree of the metal core have the same trend as described above with the etching time increases (Figures S1 and S2, Supporting Information). The high-resolution TEM images of CuO−ZnO calcined under different temperatures, especially 500 °C, show that the representative crystal lattice spacings always agree well with CuO−ZnO crystals. This result indicates that the CuZn alloy cannot be synthesized under the same calcination temperature if there is no C−N shell coating on the outside of the metal oxides (Figure S2(a-2,b-2), Supporting Information). Interestingly, a uniform distribution of the metal elements that is well matched to the core is observed in the energy-dispersive X-ray (EDX)-mapping measurement (Figure 1(g-1,g-4,g-5)). Meanwhile, an intuitive sight is obtained that the shell with a uniform distribution of C and N is well coated on the outside of the CuZn metal core (Figure 1(g-1−g-3)). This significant difference in elemental distribution is mainly because of the migration of Zn atoms from the exterior shell to the interior core of the Cu metal C

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Structural analysis in the core and shell layers of the CuZn@C−N. (a) XRD patterns of the CuZn@C−N samples involving the different molar ratio between the ligand and ZnO (the etching time is 18 h). (b) Fourier transforms of the Cu K-edge EXAFS of the CuZn@C−N sample and Cu foil, and XANES spectra of the CuZn@C−N sample and Cu foil. (c) FTIR spectra of the as-calcined PAN and the CuZn@C−N sample. (d) XPS spectra of CuZn@C−N in N 1s region. (e) CO2-TPD of the as-calcined PAN, CuZn alloy and the CuZn@C−N sample. (f) DFT calculation about the adsorption capacity for CO2 molecules in the structure of the CuZn@C−N surface containing the pyridinic N and pyrrolic N.

edge shifts to lower energy (Figure 2b). The results of XANES indicate that the 3d-band electron density of Cu has changed.30,44 The EXAFS and XANES measurements of Zn K-edge have also verified the above results (Figure S7a−c, Supporting Information).17 In addition, there is some ZnNx present in the CuZn@C−N samples, especially in the sample of high ligand content. Combined with the analysis in Figure 2a,b, the highly dispersed ZnNx nanoparticles have an excellent dispersion-strengthening effect on the CuZn alloy so that the CuZn@C−N catalyst exhibits a superior heat-transfer performance, which is considered to be beneficial to the thermal stability of the catalyst during the catalytic process.45 The surface functional groups of the as-prepared CuZn@C− N and PAN samples were investigated by FTIR spectra analysis. It is observed from Figure 2c, the O−H stretching vibration of water molecules appeared at 3450 cm−1.46 The C− H stretching vibration is matched with 2920 cm−1, and the absorption at 1630 cm−1 corresponds to CN.46,47 It is also observed that the CuZn@C−N sample and the metal-free PAN have the same structure. The formation of such a structure is mainly attributed to the decomposition and carbonization of the MOF (ZIF-8) structure at high temper-

ature (Figure S9b, Supporting Information). X-ray photoelectron spectroscopy (XPS) was used to detect the chemical compositions of the N-doped carbon shell. Figure 2d displays the deconvoluted spectrum in the N 1s region of the CuZn@ C−N samples with different molar ratios between the ligand and ZnO. In these samples, the pyridinic N (398.4 eV) is the main N species along with a small amount of pyrrolic N (400.4 eV).30,48,49 The content of pyridinic N species increases significantly as the increase of ligand content. Elemental distribution of N species in the metal-free PAN is similar to the C−N shell of the CuZn@C−N sample in addition to a slight decrease in the pyridinic N content (Figure S10c, Supporting Information). Furthermore, the increase of the calcination temperature and ligand content promotes the increase of the total N content (Table S1, Supporting Information). As shown in Figure S11a, there are three peaks appeared at 284.6, 286.0, and 288.3 eV correspond to C−C/CC, CN, and C−N, respectively.50,51 In addition, the large tail of the peak in the higher binding energy displays the significant feature of Ndoped carbon materials.52 As for the spectrum in the O 1s region (Figure S11b), the fitted peak of 533.3 eV is attributed to the adsorbed water molecules, whereas the peak appeared at D

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. RWGS catalytic performances tests of the CuZn@C−N catalysts. Comparison of the effects of the CuZn@C−N catalysts preparation conditions on CO2 conversion and CO selectivity involving (a) the different molar ratio between the ligand and ZnO (0.1 MPa, 500 °C, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3) and (b) the different etching time (0.1 MPa, 440 °C, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3). Reaction kinetic fit derived from the catalytic results of (c) different molar ratio between the ligand and ZnO (0.1 MPa, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3) and (d) different etching time (0.1 MPa, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3). It is worth noting that the etching time is 18 h when the molar ratio is investigated, and the molar ratio is fixed to 5:1 when the etching time is modulated. (e) The comparison of the catalytic activity between the as-calcined PAN, CuZn alloy and CuZn@C−N sample (0.1 MPa, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3). (f) CO2 conversion for the RWGS reaction at different reaction conditions, such as temperatures, pressure and volume ratio between CO2 and H2 (in the temperature measurement: 0.1 MPa, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3; in the pressure measurement: 400 °C, 12 000 mL h−1 gcat−1, CO2/H2 = 1:3; in the raw ratio measurement: 0.1 MPa, 400 °C).

alloy is coated by the C−N shell exhibits a significant improvement in the ability to adsorb CO2. The above results are ascribed to the Lewis basic sites, which are created by pyridinic N.55 The C atoms adjacent to pyridinic N atoms will be induced to generate more charges, providing the active sites for CO2 reduction.30,56 Moreover, the adsorbed amount of CO2 is positively correlated with the change of ligand content in the samples. However, the increase of the etching time only causes a slight increase in the adsorption amount of CO2, indicating that the improvement of the C−N content, especially the pyridinic N content, is beneficial to the adsorption of CO2 (Figure S12, Supporting Information). A DFT calculation was conducted to explore the reason for the improved activity of CO2 reduction when the CuZn alloy was coated in the N-doped carbon shell. The theoretical model including pyridinic N and pyrrolic N was constructed to obtain the difference of CO2 adsorption on the different sites. A comparison of the adsorption ability for CO2 at various sites between the pyridinic ring and pyrrolic ring is found in Figure 2f. It is observed that the pyridinic ring has more configurations for the adsorption of CO2 than the pyrrolic ring. For further clarifying the distinction of the adsorption

531.6 eV should be attributed to the function groups of hydroxyl which is adsorbed in the surface of the carbon layer.53,54 The XPS spectra in Zn 2p and Cu 2p regions can further show the detailed information involving the CuZn core. In Figure S11c, there are no satellite peaks found in the region of Cu 2p, which indicates that Cu2+ does not exist in the core. The peaks of 953 and 933 eV belong to Cu 2p1/2 and Cu 2p3/2 of metallic Cu. Furthermore, the peaks of 1044.9 and 1021.8 can be attributed to the Zn 2p1/2 and Zn 2p3/2 of metallic Zn because the distance of two peaks is 23.1 eV (Figure S11d). It should be worth noting that the binding energy in Cu 2p and Zn 2p shifts to higher value comparing with the standard value of Cu0 and Zn0, which results from the electron transfer from the CuZn core to C−N shell.30,50 The electron transfer is favorable to promote the C−O bond activation of CO2 molecules and then improve the catalytic activity. CO2 temperature-programmed desorption (CO2-TPD) gives important perspective to the ability of the samples to adsorb CO2. As shown in Figure 2e, the metal-free PAN sample with only C−N structure displays an excellent adsorption ability for CO2. Compared with the CuZn alloy, the as-synthesized CuZn@C−N sample in which the CuZn E

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

metal-free PAN, which is composed of the N-doped carbon material, displays poor catalytic activity for the RWGS reaction. Furthermore, the CuZn alloy that is not covered by a C−N material can catalyze well the transformation of CO2. Interestingly, the combination of the CuZn alloy and the C−N shell layer greatly enhanced the capacity for CO2 activation and transformation, confirming the synergistic effect of CuZn alloy and pyridinic N in the process of the RWGS reaction.1,8,16 In Figure 3f, CO2 conversion of the CuZn@ C−N catalyst for RWGS reaction is positively correlated with the reaction temperature because this catalytic process requires endotherm.10 When the temperature is lower than 350 °C, the CO2 conversion is low owing to the stable C−O bond.58 The sharp increase of the catalytic activity above 400 °C is ascribed to the effective activation of the C−O bond.58,59 The reaction pressure is also beneficial to CO2 conversion because more reactant molecules are able to contact the active sites at high pressure.58 In addition, the volume ratio of the raw gas also has an effect on the RWGS reaction. CO2 conversion is enhanced when the volume ratio between H2 and CO2 is increased, but the promotion ability will be greatly weakened when the ratio increases to over 5:1. This result derives from the large excess of H2 molecules compared with the amount required in the RWGS reaction.

sites and adsorption ability between the pyridinic ring and pyrrolic ring, the optimized adsorption structures were constructed. It is confirmed that B-1 and C-3 are the optimized adsorption configurations of the pyridinic and pyrrolic rings, respectively (Figure S13, Supporting Information). In addition, the configuration of C-1 exhibits a weak adsorption between the CO2 molecule and H atom linked to pyrrolic N, and its adsorption energy is much lower than that of other configurations. In other words, there are fewer configurations in the pyrrolic ring having an adsorption energy similar to that of the pyridinic ring. Thus, compared with the pyrrolic ring, there are more adsorption sites of CO2 in the pyridinic ring. The N atoms will promote the neighbor C atoms to present the Lewis basicity because of the induction of the electron pairs in pyridinic N atoms, and above result comes from the found that these C atoms have the local density of states in the occupied region near the Fermi level.55 In addition, the N atom of the pyridinic ring is also considered as the active site in the catalytic reaction.49,56 The key role of pyridinic N in CO2 reduction can be found from our calculation results. As observed in Figure 2f, pyridinic N (B1) contributes to the strongest adsorption (−0.26 eV) because of the presence of lone pair electrons. However, strong adsorption exists on the para-position of the N atom (−0.24 eV) on the pyrrolic ring, whereas the pyrrolic N has no adsorption, which is ascribed to the formation of p−π electron conjugation. Therefore, the lone pair of electrons in pyridinic N further enhanced the Lewis basicity, which is benefitted for adsorbing and activating of CO2 molecules. The results of the experiment are verified by the above calculation, confirming that the synergistic catalysis between the core (CuZn alloy) and the coated pyridinic N layers contributes to the excellent catalytic activity of CO2 reduction. The RWGS properties of the as-synthesized catalysts were studied according to the equation as follows: CO2 + H2 → CO + H2O ΔH° = +41.2 kJ mol−1.10,13 The effects of ligand content and etching time on reaction activity are investigated in Figure 3a,b, respectively. Obviously, more CO2 has been transformed to CO over the CuZn@C−N catalyst with the increase of ligand content, but the effect of etching time is different. In the beginning, the catalytic activity tends to decrease as the extension of etching time, whereas it has a sudden increase when the etching time reaches 18 h. To investigate the catalytic results further, the kinetics of the catalysts was explored (Figure 3c,d). The apparent activation energy (Ea) of the CuZn@C−N catalyst is significantly decreased because of the addition of more ligands. Improved dispersibility of the alloy core and increased pyridinic N content are considered as the main reasons for this result.18,24,55,57 Different etching times also show obvious difference on the apparent activation energies of the asprepared catalysts. Because the N-doped carbon shell layer is continuously thickened as the etching time is extended, it is more difficult for the reactant molecules to contact with internal active components, leading to an increase of Ea as well as a decrease of the catalytic activity. However, further extension of the etching time (18 h) caused a marked difference of Ea from the previous trend. A sudden decrease of Ea (113.9 kJ mol−1) is ascribed to the adequate N content in the shell layers, which greatly promotes the adsorption of CO2, resulting in the improvement of the catalytic activity. As for CO selectivity, it is always >99% over the different CuZn@C− N catalysts and reaction conditions. As shown in Figure 3e, the

4. CONCLUSIONS In summary, the ZnNx dispersion strengthened CuZn alloy and the uniformly covered C−N shell layers were successfully synthesized by the MOF-derived method. The core−shell catalyst in which N-doped carbon cages encapsulated the CuZn alloy achieved superior activity in the RWGS reaction even under atmospheric pressure. The CuZn alloy is considered to be the main active sites for catalyzing RWGS, which is also regarded as the active sites of methanol formation from CO2 hydrogenation in the previous reports.60,61 The synergistic catalytic effect among the shell containing pyridinic N and the alloy core contributes to significantly improving the catalytic activity of RWGS reaction. The CuZn alloy is considered as the active site, whereas pyridinic N excellently promotes the adsorption and activation of CO2. A DFT calculation further confirmed the excellent adsorption capacity of pyridinic N for CO2 molecules. This work provides a new strategy for synthesizing the dispersion strengthened alloy and gives new insight into the design of specific functionalization core−shell catalysts.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03488.

■

Detailed information about the additional XRD patterns, XPS analysis, XAFS analysis, DFT calculation, SEM images, TEM images, Brunauer−Emmett−Teller surface areas, TGA, CO2-TPD, and FTIR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.G.). *E-mail: [email protected] (W.L.). ORCID

Wei Li: 0000-0001-7287-8523 F

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

Nano-Dimension on CO2 to CO Conversion via the Reverse WaterGas Shift Reaction. Nanoscale 2017, 9, 12984−12995. (14) Schneck, F.; Schendzielorz, F.; Hatami, N.; Finger, M.; Wurtele, C.; Schneider, S. Photochemically Driven Reverse Water-Gas Shift at Ambient Conditions Mediated by a Nickel Pincer Complex. Angew. Chem., Int. Ed. 2018, 57, 14482−14487. (15) Dou, J.; Sheng, Y.; Choong, C.; Chen, L.; Zeng, H. C. Silica Nanowires Encapsulated Ru Nanoparticles as Stable Nanocatalysts for Selective Hydrogenation of CO2 to CO. Appl. Catal., B 2017, 219, 580−591. (16) Derrouiche, S.; Lauron-Pernot, H.; Louis, C. Synthesis and Treatment Parameters for Controlling metal Particle Size and Composition in Cu/ZnO MaterialsFirst Evidence of Cu3Zn Alloy Formation. Chem. Mater. 2012, 24, 2282−2291. (17) Sun, J.; Yang, G.; Ma, Q.; Ooki, I.; Taguchi, A.; Abe, T.; Xie, Q.; Yoneyama, Y.; Tsubaki, N. Fabrication of active Cu-Zn nanoalloys on H-ZSM5 zeolite for enhanced dimethyl ether synthesis via syngas. J. Mater. Chem. A 2014, 2, 8637−8643. (18) Fu, Y.; Yu, H.-Y.; Jiang, C.; Zhang, T.-H.; Zhan, R.; Li, X.; Li, J.F.; Tian, J.-H.; Yang, R. NiCo Alloy Nanoparticles Decorated on NDoped Carbon Nanofibers as Highly Active and Durable Oxygen Electrocatalyst. Adv. Funct. Mater. 2018, 28, 1705094. (19) Kuld, S.; Conradsen, C.; Moses, P. G.; Chorkendorff, I.; Sehested, J. Quantification of Zinc Atoms in a Surface Alloy on Copper in an Industrial-Type Methanol Synthesis Catalyst. Angew. Chem., Int. Ed. 2014, 53, 5941−5945. (20) Großmann, D.; Klementiev, K.; Sinev, I.; Grünert, W. Surface Alloy or Metal-Cation Interaction-The State of Zn Promoting the Active Cu Sites in Methanol Synthesis Catalysts. ChemCatChem 2017, 9, 365−372. (21) Yin, G.; Abe, H.; Kodiyath, R.; Ueda, S.; Srinivasan, N.; Yamaguchi, A.; Miyauchi, M. Selective electro- or photo-reduction of carbon dioxide to formic acid using a Cu-Zn alloy catalyst. J. Mater. Chem. A 2017, 5, 12113−12119. (22) Li, Y.; Cai, X.; Chen, S.; Zhang, H.; Zhang, K. H. L.; Hong, J.; Chen, B.; Kuo, D.-H.; Wang, W. Highly Dispersed Metal Carbide on ZIF-Derived PyridinicN-Doped Carbon for CO2 Enrichment and Selective Hydrogenation. ChemSusChem 2018, 11, 1040−1047. (23) Li, W.; Herkt, B.; Seredych, M.; Bandosz, T. J. Pyridinic-N Groups and Ultramicropore Nanoreactors Enhance CO2 Electrochemical Reduction on Porous Carbon Catalysts. Appl. Catal., B 2017, 207, 195−206. (24) Liu, S.; Yang, H.; Huang, X.; Liu, L.; Cai, W.; Gao, J.; Li, X.; Zhang, T.; Huang, Y.; Liu, B. Identifying Active Sites of NitrogenDoped Carbon Materials for the CO2 Reduction Reaction. Adv. Funct. Mater. 2018, 28, 1800499. (25) Wu, J.; Liu, M.; Sharma, P. P.; Yadav, R. M.; Ma, L.; Yang, Y.; Zou, X.; Zhou, X.-D.; Vajtai, R.; Yakobson, B. I.; Lou, J.; Ajayan, P. M. Incorporation of Nitrogen Defects for Efficient Reduction of CO2 via Two-Electron Pathway on Three-Dimensional Graphene Foam. Nano Lett. 2016, 16, 466−470. (26) Wu, J.; Yadav, R. M.; Liu, M.; Sharma, P. P.; Tiwary, C. S.; Ma, L.; Zou, X.; Zhou, X.-D.; Yakobson, B. I.; Lou, J.; Ajayan, P. M. Achieving Highly Efficient, Selective, and Stable CO2 Reduction on Nitrogen-Doped Carbon Nanotubes. ACS Nano 2015, 9, 5364−5371. (27) Li, W.; Fechler, N.; Bandosz, T. J. Chemically Heterogeneous Nitrogen Sites of Various Reactivity in Porous Carbons Provide High Stability of CO2 Electroreduction Catalysts. Appl. Catal., B 2018, 234, 1−9. (28) Leonard, N.; Ju, W.; Sinev, I.; Steinberg, J.; Luo, F.; Varela, A. S.; Roldan Cuenya, B.; Strasser, P. The Chemical Identity, State and Structure of Catalytically Active Centers during the Electrochemical CO2 Reduction on Porous Fe-Nitrogen-Carbon (Fe-N-C) Materials. Chem. Sci. 2018, 9, 5064−5073. (29) Li, W.; Seredych, M.; Rodríguez-Castellón, E.; Bandosz, T. J. Metal-free Nanoporous Carbon as a Catalyst for Electrochemical Reduction of CO2 to CO and CH4. ChemSusChem 2016, 9, 606−616. (30) Jiang, P.; Chen, J.; Wang, C.; Yang, K.; Gong, S.; Liu, S.; Lin, Z.; Li, M.; Xia, G.; Yang, Y.; Su, J.; Chen, Q. Tuning the Activity of

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21878162 and 21603107), Natural Science Foundation of Tianjin City (NSFT, 16YFZCGX00020), the Research Fund for 111 Project (B12015), and MOE (IRT-13R30 and 113016A), and the Fundamental Research Funds for the Central Universities, and dedicated to 100th anniversary of Nankai University. The authors thank the 1W1B station, Beijing Synchrotron Radiation Facility for assistance with the XAFS measurements.

■

REFERENCES

(1) Li, M. M.-J.; Zeng, Z.; Liao, F.; Hong, X.; Tsang, S. C. E. Enhanced CO2 Hydrogenation to Methanol over CuZn Nanoalloy in Ga Modified Cu/ZnO Catalysts. J. Catal. 2016, 343, 157−167. (2) Chen, G.; Gao, R.; Zhao, Y.; Li, Z.; Waterhouse, G. I. N.; Shi, R.; Zhao, J.; Zhang, M.; Shang, L.; Sheng, G.; Zhang, X.; Wen, X.; Wu, L.Z.; Tung, C.-H.; Zhang, T. Alumina-Supported CoFe Alloy Catalysts Derived from Layered-Double-Hydroxide Nanosheets for Efficient Photothermal CO2 Hydrogenation to Hydrocarbons. Adv. Mater. 2018, 30, 1704663. (3) Nordhaus, W. D. Revisiting the Social Cost of Carbon. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 1518−1523. (4) Asadi, M.; Kim, K.; Liu, C.; Addepalli, A. V.; Abbasi, P.; Yasaei, P.; Phillips, P.; Behranginia, A.; Cerrato, J. M.; Haasch, R.; Zapol, P.; Kumar, B.; Klie, R. F.; Abiade, J.; Curtiss, L. A.; Salehi-Khojin, A. Nanostructured Transition Metal Dichalcogenide Electrocatalysts for CO2 Reduction in Ionic Liquid. Science 2016, 353, 467−470. (5) Liang, X.-L.; Dong, X.; Lin, G.-D.; Zhang, H.-B. Carbon Nanotube-Supported Pd−ZnO Catalyst for Hydrogenation of CO2 to Methanol. Appl. Catal., B 2009, 88, 315−322. (6) Urbain, F.; Tang, P.; Carretero, N. M.; Andreu, T.; Gerling, L. G.; Voz, C.; Arbiol, J.; Morante, J. R. A Prototype Reactor for Highly Selective Solar-Driven CO2 Reduction to Synthesis Gas Using Nanosized Earth-Abundant Catalysts and Silicon Photovoltaics. Energy Environ. Sci. 2017, 10, 2256−2266. (7) Lin, S.; Diercks, C. S.; Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C. J. Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water. Science 2015, 349, 1208−1213. (8) Ju, W.; Bagger, A.; Hao, G. P.; Varela, A. S.; Sinev, I.; Bon, V.; Roldan Cuenya, B.; Kaskel, S.; Rossmeisl, J.; Strasser, P. Understanding Activity and Selectivity of Metal-Nitrogen-Doped Carbon Catalysts for Electrochemical Reduction of CO2. Nat. Commun. 2017, 8, 944. (9) Kattel, S.; Liu, P.; Chen, J. G. Tuning Selectivity of CO2 Hydrogenation Reactions at the Metal/Oxide Interface. J. Am. Chem. Soc. 2017, 139, 9739−9754. (10) Jia, J.; Qian, C.; Dong, Y.; Li, Y. F.; Wang, H.; Ghoussoub, M.; Butler, K. T.; Walsh, A.; Ozin, G. A. Heterogeneous Catalytic Hydrogenation of CO2 by Metal Oxides: Defect EngineeringPerfecting Imperfection. Chem. Soc. Rev. 2017, 46, 4631−4644. (11) Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Sanz, J. F.; Rodriguez, J. A. Highly Active Copper-Ceria and Copper-CeriaTitania Catalysts for Methanol Synthesis from CO2. Science 2014, 345, 546−550. (12) Nielsen, D. U.; Hu, X.-M.; Daasbjerg, K.; Skrydstrup, T. Chemically and Electrochemically Catalysed Conversion of CO2 to CO with Follow-Up Utilization to Value-Added Chemicals. Nat. Catal. 2018, 1, 244−254. (13) Fishman, Z. S.; He, Y.; Yang, K. R.; Lounsbury, A. W.; Zhu, J.; Tran, T. M.; Zimmerman, J. B.; Batista, V. S.; Pfefferle, L. D. Hard Templating Ultrathin Polycrystalline Hematite Nanosheets: Effect of G

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Carbon for Electrocatalytic Hydrogen Evolution via an IridiumCobalt Alloy Core Encapsulated in Nitrogen-Doped Carbon Cages. Adv. Mater. 2018, 30, 1705324. (31) Shen, Y.; Zhou, Y.; Wang, D.; Wu, X.; Li, J.; Xi, J. NickelCopper Alloy Encapsulated in Graphitic Carbon Shells as Electrocatalysts for Hydrogen Evolution Reaction. Adv. Energy Mater. 2018, 8, 1701759. (32) Hu, X.; Qin, W.; Guan, Q.; Li, W. The Synergistic Effect of CuZnCeOx in Controlling the Formation of Methanol and CO from CO2 Hydrogenation. ChemCatChem 2018, 10, 4438−4449. (33) Jayachandrababu, K. C.; Verploegh, R. J.; Leisen, J.; Nieuwendaal, R. C.; Sholl, D. S.; Nair, S. Structure Elucidation of Mixed-Linker Zeolitic Imidazolate Frameworks by Solid-State 1H CRAMPS NMR Spectroscopy and Computational Modeling. J. Am. Chem. Soc. 2016, 138, 7325−7336. (34) Lei, Z.; Deng, Y.; Wang, C. Multiphase Surface Growth of Hydrophobic ZIF-8 on Melamine Sponge for Excellent Oil/Water Separation and Effective Catalysis in a Knoevenagel Reaction. J. Mater. Chem. A 2018, 6, 3258−3263. (35) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles. Science 2008, 322, 932−934. (36) Xie, M.; Ai, S.; Yang, J.; Yang, Y.; Chen, Y.; Jin, Y. In-Situ Generation of Oxide Nanowire Arrays from AgCuZn Alloy Sulfide with Enhanced Electrochemical Oxygen-Evolving Performance. ACS Appl. Mater. Interfaces 2015, 7, 17112−17121. (37) Yu, H.; Fisher, A.; Cheng, D.; Cao, D. Cu,N-codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8, 21431− 21439. (38) Kuang, M.; Wang, Q.; Han, P.; Zheng, G. Cu, Co-Embedded N-Enriched Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions. Adv. Energy Mater. 2017, 7, 1700193. (39) Hu, M.; Zhao, S.; Liu, S.; Chen, C.; Chen, W.; Zhu, W.; Liang, C.; Cheong, W.-C.; Wang, Y.; Yu, Y.; Peng, Q.; Zhou, K.; Li, J.; Li, Y. MOF-Confined Sub-2 nm Atomically Ordered Intermetallic PdZn Nanoparticles as High-Performance Catalysts for Selective Hydrogenation of Acetylene. Adv. Mater. 2018, 30, 1801878. (40) Zhu, Y.; Sow, C.-H.; Yu, T.; Zhao, Q.; Li, P.; Shen, Z.; Yu, D.; Thong, J. T.-L. Co-synthesis of ZnO-CuO Nanostructures by Directly Heating Brass in Air. Adv. Funct. Mater. 2006, 16, 2415−2422. (41) Yang, N.; Zhang, Z.; Chen, B.; Huang, Y.; Chen, J.; Lai, Z.; Chen, Y.; Sindoro, M.; Wang, A.-L.; Cheng, H.; Fan, Z.; Liu, X.; Li, B.; Zong, Y.; Gu, L.; Zhang, H. Synthesis of Ultrathin PdCu Alloy Nanosheets Used as a Highly Efficient Electrocatalyst for Formic Acid Oxidation. Adv. Mater. 2017, 29, 1700769. (42) Nam, D.-H.; Bushuyev, O. S.; Li, J.; De Luna, P.; Seifitokaldani, A.; Dinh, C.-T.; García de Arquer, F. P.; Wang, Y.; Liang, Z.; Proppe, A. H.; Tan, C. S.; Todorović, P.; Shekhah, O.; Gabardo, C. M.; Jo, J. W.; Choi, J.; Choi, M.-J.; Baek, S.-W.; Kim, J.; Sinton, D.; Kelley, S. O.; Eddaoudi, M.; Sargent, E. H. Metal-Organic Frameworks Mediate Cu Coordination for Selective CO2 Electroreduction. J. Am. Chem. Soc. 2018, 140, 11378−11386. (43) Chao, T.; Luo, X.; Chen, W.; Jiang, B.; Ge, J.; Lin, Y.; Wu, G.; Wang, X.; Hu, Y.; Zhuang, Z.; Wu, Y.; Hong, X.; Li, Y. Atomically Dispersed Copper-Platinum Dual Sites Alloyed with Palladium Nanorings Catalyze the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2017, 56, 16047−16051. (44) Mao, J.; Chen, W.; Sun, W.; Chen, Z.; Pei, J.; He, D.; Lv, C.; Wang, D.; Li, Y. Rational Control of the Selectivity of a Ruthenium Catalyst for Hydrogenation of 4-Nitrostyrene by Strain Regulation. Angew. Chem., Int. Ed. 2017, 56, 11971−11975. (45) Yin, Y.; Hu, B.; Li, X.; Zhou, X.; Hong, X.; Liu, G. Pd@Zeolitic Imidazolate Framework-8 Derived PdZn Alloy Catalysts for Efficient Hydrogenation of CO2 to Methanol. Appl. Catal., B 2018, 234, 143− 152.

(46) Nie, R.; Jiang, H.; Lu, X.; Zhou, D.; Xia, Q. Highly active electron-deficient Pd clusters on N-doped active carbon for aromatic ring hydrogenation. Catal. Sci. Technol. 2016, 6, 1913−1920. (47) Guan, Z.; Liu, H.; Xu, B.; Hao, X.; Wang, Z.; Chen, L. GelatinPyrolyzed Mesoporous Carbon as a High-Performance SodiumStorage Material. J. Mater. Chem. A 2015, 3, 7849−7854. (48) Wang, S.; Shang, L.; Li, L.; Yu, Y.; Chi, C.; Wang, K.; Zhang, J.; Shi, R.; Shen, H.; Waterhouse, G. I. N.; Liu, S.; Tian, J.; Zhang, T.; Liu, H. Metal-Organic-Framework-Derived Mesoporous Carbon Nanospheres Containing Porphyrin-Like Metal Centers for Conformal Phototherapy. Adv. Mater. 2016, 28, 8379−8387. (49) Li, J.-S.; Wang, Y.; Liu, C. H.; Li, S. L.; Wang, Y. G.; Dong, L. Z.; Dai, Z. H.; Li, Y. F.; Lan, Y. Q. Coupled Molybdenum Carbide and Reduced Graphene Oxide Electrocatalysts for Efficient Hydrogen Evolution. Nat. Commun. 2016, 7, 11204. (50) Zhao, Q.; Liu, L.; Liu, R.; Zhu, L. PdCu Nanoalloy Immobilized in ZIF-Derived N-doped Carbon/Graphene Nanosheets: Alloying Effect on Catalysis. Chem. Eng. J. 2018, 353, 311− 318. (51) Cheng, J.; Liu, N.; Hu, L.; Li, Y.; Wang, Y.; Zhou, J. Polyethyleneimine Entwine Thermally-Treated Zn/Co Zeolitic Imidazolate Frameworks to Enhance CO2 Adsorption. Chem. Eng. J. 2019, 364, 530−540. (52) Huang, M.; Mi, K.; Zhang, J.; Liu, H.; Yu, T.; Yuan, A.; Kong, Q.; Xiong, S. MOF-Derived Bi-metal Embedded N-doped Carbon Polyhedral Nanocages with Enhanced Lithium Storage. J. Mater. Chem. A 2017, 5, 266−274. (53) Ding, D.; Shen, K.; Chen, X.; Chen, H.; Chen, J.; Fan, T.; Wu, R.; Li, Y. Multi-Level Architecture Optimization of MOF-Templated Co-Based Nanoparticles Embedded in Hollow N-Doped Carbon Polyhedra for Efficient OER and ORR. ACS Catal. 2018, 8, 7879− 7888. (54) Cui, J.; Wang, L.; Han, Y.; Liu, W.; Li, Z.; Guo, Z.; Hu, Y.; Chang, Z.; Yuan, Q.; Wang, J. ZnO nano-cages derived from ZIF-8 with enhanced anti mycobacterium-tuberculosis activities. J. Alloys Compd. 2018, 766, 619−625. (55) Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science 2016, 351, 361−365. (56) Lv, Q.; Si, W.; He, J.; Sun, L.; Zhang, C.; Wang, N.; Yang, Z.; Li, X.; Wang, X.; Deng, W.; Long, Y.; Huang, C.; Li, Y. Selectively Nitrogen-Doped Carbon Materials as Superior Metal-Free Catalysts for Oxygen Reduction. Nat. Commun. 2018, 9, 3376. (57) Prieto, G.; Zečević, J.; Friedrich, H.; de Jong, K. P.; de Jongh, P. E. Towards Stable Catalysts by Controlling Collective Properties of Supported Metal Nanoparticles. Nat. Mater. 2013, 12, 34−39. (58) Xu, H.; Li, Y.; Luo, X.; Xu, Z.; Ge, J. Monodispersed Gold Nanoparticles Supported on a Zirconium-Based Porous MetalOrganic Framework and Their High Catalytic Ability for the Reverse Water-Gas Shift Reaction. Chem. Commun. 2017, 53, 7953−7956. (59) Zhang, X.; Zhu, X.; Lin, L.; Yao, S.; Zhang, M.; Liu, X.; Wang, X.; Li, Y.-W.; Shi, C.; Ma, D. Highly Dispersed Copper over β-Mo2C as an Efficient and Stable Catalyst for the Reverse Water Gas Shift (RWGS) Reaction. ACS Catal. 2016, 7, 912−918. (60) Behrens, M.; Studt, F.; Kasatkin, I.; Kuhl, S.; Havecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B.-L.; Tovar, M.; Fischer, R. W.; Norskov, J. K.; Schlogl, R. The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 336, 893−897. (61) Kuld, S.; Thorhauge, M.; Falsig, H.; Elkjaer, C. F.; Helveg, S.; Chorkendorff, I.; Sehested, J. Quantifying the Promotion of Cu Catalysts by ZnO for Methanol Synthesis. Science 2016, 352, 969− 974.

H

DOI: 10.1021/acsami.9b03488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX