Preparation of hierarchical hollow structures assembled from porous NiCo2O4 nanosheets for diesel soot elimination

Construction of catalysts with designed microstructures is critically important in heterogeneous catalysis. The soot catalytic oxidation was considered as one of the most efficient methods for the diesel soot elimination, but its development has been hindered by the lack of efficient catalysts. The hierarchically porous materials with well-defined microstructures are expected to perform high-catalytic activity for the large-sized soot particulates elimination. Herein, we report a method to prepare a novel hierarchical hollow structure (HHS) assembled from porous NiCo 2 O 4 NSs, referred to as NiCo 2 O 4 HHS. The cation-exchange reaction and Kirkendall effect induce the conversion from the Co-Asp (Asp: aspartic acid) fibers to the Co,Ni-Asp HHSs. The NiCo 2 O 4 HHSs are prepared through annealing the Co,Ni-Asp HHSs. As a proof-of-concept application, the unique NiCo 2 O 4 HHS is used as a catalyst to eliminate the soot particulates and exhibits superior performance, which could be attributed to the improved soot-catalyst contact, sufficient active sites and strong active oxygen species.


| INTRODUCTION
2][3][4] Soot particulates released from diesel engines give rise to serious environmental and health problems. 5,6The combination of diesel particulate filters and soot oxidation catalysts has been considered as one of the most efficient methods for the diesel soot elimination. 7,8Therefore, it is desired to develop low-cost, efficient and green catalysts to decrease the ignition temperature of soot particulates.
Although platinum-based precious metals have shown excellent catalytic performance, the scarcity and highprice limit their broad utilization. 9,10Recently, numerous cost-effective transition metal oxides, especially those with spinel-like or perovskite-like structures, [11][12][13] have been explored and used as soot combustion catalysts.However, the catalytic oxidation of soot particulates is a typical gas-solid-solid tri-phase reaction consisting of gaseous reactants (NO x and O 2 ), solid reactants (soot particulates) and solid catalyst.The conventional meso-/microporous catalysts are inaccessible for the large-sized soot particulates (25-100 nm). 14To solve this problem, a series of advanced catalysts with macro-porous structures have been developed.For example, by using the hard-template method, Wei et al. synthesized three-dimensional (3D) ordered macro-porous catalysts, which exhibited improved catalytic activity for the soot combustion. 15In addition, our previous works demonstrated that the monolithic nanoarray structure could remarkably improve the soot-catalyst contact. 16Nevertheless, the relatively small surface area of macro-porous catalysts limits the number of active sites.Therefore, the hierarchically porous materials are expected to perform high-catalytic activity.On the other hand, although two-dimensional (2D) nanosheets (NSs) exhibited high-catalytic performance, [17][18][19][20] their easy aggregation hinders their wide utilization.Fortunately, the assembly of 2D NSs into hollow structures can prevent 2D NSs from aggregation, endowing 2D NSs with promising potential in catalysis. 21,22n this work, we report a method to prepare a novel hierarchical hollow structure (HHS) assembled from porous NiCo 2 O 4 NSs, referred to as NiCo 2 O 4 HHS.As a proof-of-concept application, the unique NiCo 2 O 4 HHS is used as a catalyst to eliminate the soot particulates, which exhibits superior performance.
Figure 1A illustrates the synthetic procedure for the porous NiCo 2 O 4 HHSs.4][25] The Co,Ni-Asp HHSs made of NSs were obtained by reacting Co-Asp fibers with Ni 2+ ions for 3 hours (Step 1 in Figure 1A).The scanning electron microscope (SEM) images show the hierarchical nanostructure assembled from 2D NSs with thickness of 12.1 ± 3.4 nm (Figure 1B and Figure S3A).The transmission electron microscope (TEM) image of a single nanostructure reveals its hollow interior and open ends (Figure 1C), indicating the formation of 2D NS-assembled hollow structure.The X-ray diffraction (XRD) pattern confirms the inorganic-organic hybrid crystal structure of Co,Ni-Asp HHSs (Figure S3B).7][28][29] As shown in the SEM images (Figure 1E and its inset), although the hierarchical morphology of NS-based hollow structures maintained well after annealing, mesopores appeared in the NSs with thickness of 10.9 ± 1.9 nm (Figure S4A).The TEM image (Figure 1F) confirms the 2D NS-based hollow structure with open ends.A typical STEM-EDS elemental mapping manifests the homogeneous distribution of Co, Ni, and O in the thermal-induced decomposition product (Figure 1G).The internal cavities are further verified by the STEM-EDS elemental line scanning profiles (Figure 1H).The surface area and size distribution of mesopores (2-50 nm) in the product are determined by nitrogen physisorption.The mesopores with pore size centered at ~7 nm (inset in Figure 1I) were generated in the NSs owing to the thermal decomposition of aspartic acid, providing a specific surface area as high as 213.6 m 2 /g (Figure 1I).Moreover, the hierarchical structure assembled from 2D NSs endows the NiCo 2 O 4 HHSs with numerous macropores with pore size of 89.7 ± 49.0 nm formed on the surface of HHSs (Figure 1E and Figure S4B).All the XRD peaks can be indexed to the spinel-structured NiCo 2 O 4 (JCPDS No. 20-0781) (Figure 1J).In the Ni 2p X-ray photoelectron spectroscopy (XPS) spectrum (Figure S5A), the peaks at 873.9 and 855.8 eV are assigned to Ni 3+ , while the binding energy at 871.6 and 854.1 eV are indexed to Ni 2+ .In the Co 2p spectrum (Figure S5B), the peaks at 795.1 and 780.1 eV are attributed to Co 3+ , whereas, the peaks centered at 796.7 and 782.8 eV arise from Co 2+ .All the aforementioned results prove the formation of the NiCo 2 O 4 HHSs.
To reveal the conversion from Co-Asp fibers to Co,Ni-Asp HHSs, the intermediates obtained at different reaction time were studied by SEM and TEM (Figure 2).As shown in Figure 2A,B 2E,F). 30hen the interior Co-Asp is completely consumed, the Co,Ni-Asp HHS is obtained (Figure 2G,H). 31s a proof-of-concept application, the Co,Ni-Asp HHSs were used as catalyst to eliminate the soot particulates.The catalytic soot combustion was evaluated by temperature programmed oxidation (TPO).As comparison, the disorder NiCo 2 O 4 NSs were prepared (Figure S6) 32,33 and also used as catalyst for the soot combustion.As shown in Figure 3A, the NiCo 2 O 4 HHSs exhibit higher activity than NiCo 2 O 4 NSs.Compared with the NiCo 2 O 4 NSs, the T 10 value, that is, the temperature at 10% soot conversion, decreases from 330 C to 321 C using NiCo 2 O 4 HHSs as catalyst, and the T 50 value, that is, the temperature at 50% soot conversion, decreases from 379 C to 354 C (Figure 3B), which is among the best in the soot elimination catalysts (Table S1).It is worth mentioning that the practical temperature when diesel engines exhaust is in the range of 150 C to 380 C. 34 Therefore, the NiCo 2 O 4 HHSs can catalytically remove soot particulates at the practical exhaust temperature.As known, NO 2 is a strong oxidizer and can be reduced by soot particulates.35,36 In order to reveal the role of NO x (NO + NO 2 ) in combustion of soot particulates, the NO x concentration was in situ monitored.As a result, the NO concentration decreases more (Figure 3C), and the NO 2 concentration increases more using the NiCo 2 O 4 HHSs as catalyst compared to the NiCo 2 O 4 NSs (Figure 3D), revealing that the NiCo 2 O 4 HHSs can more efficiently catalyze the oxidation of NO to NO 2 .It is worth mentioning that the temperature corresponding to the maximum NO 2 concentration (319 C, Figure 3C,D) is very close to the ignation temperature of soot using NiCo 2 O 4 HHSs (321 C, Figure 3B).Therefore, it is reasonable to indicate that the removal of soot is accompanied by the reduction of NO 2 .
Based on the aforementioned results, the soot oxidation process using the NiCo 2 O 4 HHSs is illustrated (Figure 4A).Soot particulates are first captured by the 2D NSs of NiCo 2 O 4 HHSs (Step 1 in Figure 4A and Figure S7).After NO is oxidized to NO 2 on the surface of illustration for preparation of the NiCo 2 O 4 HHS.B, SEM image, C, TEM image, and D, STEM-EDS elemental mappings of the Co,Ni-Asp HHSs.Inset in (B): highmagnification SEM image of Co,Ni-Asp HHSs.E, SEM image, F, TEM image, G, STEM-EDS elemental mappings in the selected area in (F), H, STEM-EDS elemental line scanning, I, nitrogen adsorption/ desorption isotherms, and J, XRD pattern of the NiCo 2 O 4 HHSs.Insets in (E): high-magnification SEM image, and (I), mesopore size distribution of the NiCo 2 O 4 HHSs (STEM-EDS) elemental mapping images exhibit that the elements of Co, Ni, and O are uniformly distributed in the whole hollow structure (Figure 1D).The annealing treatment can induce the decomposition of Co,Ni-Asp HHSs to form porous NiCo 2 O 4 HHSs (Step 2 in Figure , the surface of Co-Asp fibers is quite smooth.After the Co-Asp fibers reacted with Ni 2+ ions for 0.5 hour, a thin layer of small NSs were generated on the surface (Figure 2C,D).A Co-Asp fiber@Co,Ni-Asp NS yolk-shell heterostructure was obtained at reaction time of 1 hour (Figure 2E,F).When the reaction was stopped at 3 hours, the Co-Asp fibers have been fully converted into the Co,Ni-Asp HHSs (Figure 2G,H).A possible mechanism involving the cation-exchange reaction and Kirkendall effect is proposed for formation of the Co,Ni-Asp HHSs (Figure 2I).

F
I G U R E 2 A, C, E, G, SEM, and B, D, F, H, TEM images of the obtained products after the Co-Asp fibers reacted with Ni 2+ at different reaction time: A and B, 0 hour, C and D, 0.5 hour, E and F, 1 hour, and G and H, 3 hours.I, Schematic illustration of the conversion from the Co-Asp fiber to the Co,Ni-Asp HHS First, Ni 2+ ions can quickly exchange with Co 2+ ions of Co-Asp fibers (Figure 2A,B) to produce a thin layer of Co,Ni-Asp shell on the surface of the Co-Asp fiber (Figure 2C,D).The concentration gradient of metal ions can drive the outward diffusion of Co 2+ ions and the inward diffusion of Ni 2+ ions.The difference of the diffusion rates between Co 2+ and Ni 2+ ions (ie, Kirkendall effect) lead to the yolk-shell intermediate (Figure Catalytic soot combustion activities of NiCo 2 O 4 HHSs and NiCo 2 O 4 NSs.B, T 10 and T 50 values of soot oxidation using NiCo 2 O 4 HHSs and NiCo 2 O 4 NSs as catalysts.C, Concentration profiles of NO using NiCo 2 O 4 HHSs and NiCo 2 O 4 NSs as catalysts.D, Profiles of NO 2 concentrations and soot removal rate using NiCo 2 O 4 HHSs and NiCo 2 O 4 NSs as catalysts NiCo 2 O 4 HHSs, both NO 2 and O 2 oxidize the soot into CO 2 (Step 2 in Figure 4B).To further explain the high activity of the NiCo 2 O 4 HHSs, XPS and temperature programmed reduction by soot (soot-TPR) were performed.The O 1s peak in the XPS (Figure 4B and Figure S8) showed that the oxygen adsorbed on the surface of the NiCo 2 O 4 HHSs are much more compared to that of the NiCo 2 O 4 NSs.Moreover, there are mainly two reductive peaks of α and β in the soot-TPR (Figure 4C).The α peak represents the active oxygen species for the soot elimination.Compared with the NiCo 2 O 4 NSs, the α peak in the NiCo 2 O 4 HHSs appears at lower temperature, indicating its stronger oxidation ability.Moreover, there is no obvious change of the morphology, structure and chemical state of the NiCo 2 O 4 HHSs after the TPO test (Figure 1E,J, Figures S5 and S9), indicating their high stability.Therefore, the high activity and stability for the catalytic soot elimination can be ascribed to the unique hierarchical nanostructure of the NiCo 2 O 4 HHSs.The macropores generated from the assembled 2D NSs on the surface of NiCo 2 O 4 HHSs can capture the soot particulates to improve the soot-catalyst contacts.The mesopores generated in the NSs, owing to the thermal decomposition of aspartic acid, provide high-surface area and abundant active sites.The 2D NSs are endowed with strong active oxygen species absorbed on their surface, and the hollow architectures effectively avoid the aggregation of 2D NSs during catalytic process.In summary, we have successfully synthesized the NiCo 2 O 4 HHSs.The cation-exchange reaction and Kirkendall effect induce the conversion from the Co-Asp fibers to the Co,Ni-Asp HHSs.The NiCo 2 O 4 HHSs are prepared through annealing the Co,Ni-Asp HHSs.The as-obtained NiCo 2 O 4 HHSs exhibit high activity for the catalytic elimination of soot particulates, which could be attributed to the improved soot-catalyst contact, sufficient active sites and strong active oxygen species. 2 | EXPERIMENTAL SECTION 2.1 | Synthesis of Co-aspartic acid (Co-Asp) fibersIn a typical procedure, 3 mmol of Co(NO 3 ) 2 Á6H 2 O, 3 mmol of L-aspartic acid, and 3 mL of NaOH (2 M) aqueous solution were added to a mixture of 15-mL deionized water and 18-mL glycol.After vigorous stirring for 1 hour, the solution was transferred into a 50-mL Teflon-lined autoclave, and then heated and maintained at 180 C for 5 hours.The pink precipitate was collected by centrifugation at 3000 rpm for 10 minutes, and dried at 40 C for 12 hours.

2. 2 |
Synthesis of the Co,Ni-Asp hierarchical hollow structures (Co,Ni-Asp HHSs) The as-prepared Co-Asp fibers (0.075 g) were dispersed in 20 mL ethanol.Then, 10 mL of Ni(NO 3 ) 2 Á6H 2 O F I G U R E 4 A, Schematic illustration of the soot capture and elimination using the NiCo 2 O 4 HHS as catalyst.B, O 1s binding energy (BE) and percentages of different surface oxygen species.C, Soot-TPR results using NiCo 2 O 4 HHSs and NiCo 2 O 4 NSs as catalysts