[email protected]2O3-IE surrounded catalysts with core-shell structure (for CO methanation) prepared by IEIL method
The catalytic hydrogenation of carbon oxide (CO or CO2) to produce synthetic natural gas (SNG), known as methanation, is an indispensable process of coal-to-SNG technology in industry. The methanation catalyst should have both high activity and good stability at high temperatures (since the methanation process is an exothermic reaction). Conventional Ni/Al2O3 catalysts prepared through the traditional impregnation method usually suffer from rapid deactivation as a result of serious carbon deposition and the sintering of Ni particles during the methanation process (27). Thus, developing an efficient methanation catalyst with simultaneously smaller particle size to resist the coking and high thermal stability to avoid the sintering of Ni particles is highly challenging (28). Figure 2A shows the schematic diagram toward the synthesis of core-shell structured [email protected]2O3-IE catalyst by the proposed IEIL strategy. For comparison, conventional Ni/Al2O3-IM supported catalyst is also prepared by impregnation (IM) method.
Figure 2 (B to E) shows the morphology evolution from the as-prepared nanosheet Ni(OH)2 precursor to the final core-shell [email protected]2O3-IE catalyst. The nickel hydroxide nanosheets process a thickness of ~15 nm (Fig. 2B). After IE reaction with Al3+ and the following calcination, the resulted Ni(OH)2@Al(OH)3 MMH (NiAl-MMH) (Fig. 2C) and [email protected]2O3-IE (Fig. 2D) inherit the nanosheet morphology, respectively. After further reduction of [email protected]2O3-IE by H2, the obtained [email protected]2O3-IE (Fig. 2E) with nickel loading content of 16 weight % (wt %) (as a typical representative) processes a peapod-like core-shell structure with the nickel nanoparticles (ca. 2.9 ± 0.5 nm) surrounded by an alumina substrate [as shown in the enlarged transmission electron microscopy (TEM) image in Fig. 2F]. The IEIL method shows great advantages for the synthesis of high-loading (even up to 44 wt % Ni) Ni-Al2O3 catalyst with highly dispersed Ni nanoparticles (see fig. S1A and section S1). In sharp contrast, the Ni/Al2O3-IM sample with the same Ni loading (16 wt %) (see fig. S2A and section S2) prepared by the traditional impregnation method exhibits a random packing of Ni nanoparticles with a very broad particle size distribution (7 to 13 nm) supported on the surface of Al2O3. On the basis of the H2 pulse adsorption, the Ni dispersion of [email protected]2O3-IE is 15.0%, three times higher than that of Ni/Al2O3-IM (4.6%), which is in good agreement with the much smaller Ni particle size in [email protected]2O3-IE.
Figure 2G shows the x-ray diffraction (XRD) patterns of the samples from a nickel hydroxide precursor to [email protected]2O3-IE. To demonstrate the structure evolution, a schematic diagram of the formation process of [email protected]2O3-IE is proposed in Fig. 3 (A to D). After ion-exchange reaction for a certain period, the obtained MMH precursors (NiAl-MMH) are Ni-Al hydrotalcite-like compounds (as shown in Fig. 2G), in which Ni2+ is located mainly in the center while the surface of NiAl-MMH is enriched by Al3+, forming a Ni(OH)2@Al(OH)3-like gradient structure due to the incomplete IE process (Fig. 3A). After calcination, three new peaks emerged in the calcined [email protected]2O3-IE (as shown in Fig. 2G) can be assigned to NiO, which shifted to higher 2θ values than those of pure NiO due to the diffusion of Al3+ in NiO (29), forming a mixed oxide (NiAl-MO) phase (Fig. 3B). During the reduction process by H2, Ni2+ is reduced to form metallic Ni centers accompanied by the migration of Ni2+ from NiAl-MO phase, resulting in the crystallization of residual alumina (Fig. 3, C and D). However, the reduction of Ni2+ in the NiAl-MO phase is generally hindered by alumina, and as a consequence, the residual Ni2+ in Al2O3 shifts the reflections of Al2O3 to lower 2θ values compared to those of the commercial pure γ-Al2O3 and Ni/Al2O3-IM as shown in Fig. 2G. In contrast, the migration of Ni2+ to γ-Al2O3 (Fig. 3E) in Ni/Al2O3-IM prepared by impregnation is ignorable (Fig. 3, F and G) because of the weak interface interaction between Ni precursor and the inert γ-Al2O3 support. The obvious differences both in structure and composition (as shown in Fig. 3, D and G) highlight the unique features and advantages of the surrounded catalyst prepared by IEIL method with active core surrounded structure and mutually (active core and support) changed interfaces (generally resulting in stronger interaction) compared to the normal supported catalyst prepared by the traditional methods.
The nitrogen sorption isotherm of [email protected]2O3-IE displays a type IV isotherm with a H2-type hysteresis loop at a relative pressure (P/P0) of 0.4 to 0.6 (Fig. 3H), revealing the presence of mesoporous structure. The specific Brunauer-Emmett-Teller surface area of [email protected]2O3-IE is 184 m2 g−1, and the average pore size calculated by the Barrett-Joyner-Halenda method is around 3.7 nm (Fig. 3H, inset). In a typical synthesis, after IE process and calcination, core-shell structured [email protected]2O3-IE mixed metal oxide precursor can be obtained. During the subsequent reduction by H2, the Al2O3 shell basically remains unchanged, while NiO is reduced to metallic Ni accompanied with the volume shrinkage, leading to abundant porosity that is responsible for the mesopores existed in [email protected]2O3-IE (Fig. 3H). Therefore, the average pore diameter of 3.7 nm corresponds to the average particle size of the original NiO encapsulated by Al2O3 shell. On the basis of the XRD results, when cubic NiO is reduced to cubic Ni, the cell volume shrinks by 40% theoretically. According to the theoretical calculation, NiO with diameter of 3.7 nm will shrink to 3.1 nm after reduction to Ni by assuming spherical shape of NiO and Ni particles. The calculated result (3.1 nm) accords well with the mean particle size of metal nickel (2.9 nm) measured by TEM, confirming the formation of york-shell–like structure of [email protected]2O3-IE. In sharp contrast, the nitrogen sorption isotherm of Ni/Al2O3-IM exhibits a H3-type hysteresis at a relative pressure (P/P0) of 0.7 to 1.0 (see fig. S2B and section S2), indicating the presence of slit-like pore that is caused by the stacking of Al2O3 support. In addition, the pore size distribution is broad (mean pore size of 29.4 nm) because of the disordered stacking of Al2O3 support particles (fig. S2B).
On the basis of the CO temperature-programmed desorption (CO-TPD) results (fig. S2C), [email protected]2O3-IE presents strong CO desorption peaks, indicating the feasible CO adsorption on Ni surface. In addition, CO desorption temperature of the [email protected]2O3-IE surrounded catalyst shifts toward lower temperature compared with the traditional Ni/Al2O3-IM supported catalyst, demonstrating that CO molecules are easier to be desorbed on [email protected]2O3-IE, which may be attributed to the confinement effect of the core-shell structure of [email protected]2O3-IE (30, 31).
The reducibility of nickel species in [email protected]2O3-IE are further investigated by H2 temperature-programmed reduction (H2-TPR) (Fig. 3I). Three hydrogen consumption peaks denoted as α-, β-, and γ-type can be assigned to the reduction of NiO with small particle size, less reducible NiO in MO phase, and stable nickel aluminate phase with the spinel structure, respectively (32). Compared to Ni/Al2O3-IM, α-type NiO in [email protected]2O3-IE is obviously more active and the reduction temperature decreases from 450 to ~300°C, implying the presence of highly dispersed NiO species with smaller particle size, which is responsible for the excellent low-temperature catalytic activity. The β-type NiO is dominant in [email protected]2O3-IE, and the peak shifts toward higher temperature, suggesting that the interaction between NiO and Al2O3 is stronger in the NiAl-MO phase than that in Ni/Al2O3-IM, confirming the mutually (NiO and Al2O3) changed interfaces in [email protected]2O3-IE. The Ni2+ contribution in x-ray photoelectron spectra (XPS) (Fig. 3J) shifts to higher binding energy in [email protected]2O3-IE (855.3 eV) than that in Ni/Al2O3-IM (854.7 eV); simultaneously, the peak assigned to Al3+ in Al2O3 (Fig. 3K) shifts toward lower binding energy in [email protected]2O3-IE (74.5 eV) than that in Ni/Al2O3-IM (74.9 eV), further confirming the strong interaction between Ni species and alumina in [email protected]2O3-IE.
The catalytic performances of [email protected]2O3-IE and Ni/Al2O3-IM toward CO methanation reaction are shown in Fig. 4 (A to D) and fig. S3. The value of T100, corresponding to the temperature at which 100% conversion is obtained, is 380°C for Ni/Al2O3-IM, while the CO conversion on [email protected]2O3-IE exceeds 90% at 260°C and reaches 100% at 280°C (Fig. 4A). On the basis of the results of H2 chemisorption and the CO conversion at 280°C under a high weight hourly space velocity (WHSV) of 100,000 ml g−1 hour−1, the turnover frequency (TOF)CO,280 values of [email protected]2O3-IE is 3.7 s−1, six times higher than that of Ni/Al2O3-IM (0.6 s−1), further confirming the superior catalytic activity of the [email protected]2O3-IE catalyst. In the lifetime test, the [email protected]2O3-IE catalyst exhibits simultaneously high stability and high yield (without obvious decline) during 100 hours at 500°C with a high space velocity of 100,000 ml g−1 hour−1 (Fig. 4, B and D). In sharp contrast, the activity of the Ni/Al2O3-IM has a remarkable decline after only 30 hours, and the reaction had to be stopped because of the serious plugging of the reactor by coke. The carbon content of the used catalysts after CO methanation is 4.9 and 17.5 wt % for [email protected]2O3-IE-100h and Ni/Al2O3-IM-30h, respectively, indicating much better anti-coking property of [email protected]2O3-IE (see fig. S4A and section S4). The NH3-TPD results (see fig. S2D and section S2) of [email protected]2O3-IE indicate that the strong interaction of Ni and Al2O3 lowers the acid amount, which may contribute to the anti-coking performance. The morphology and crystallinity of the used catalyst are characterized and analyzed (see fig. S4 and section S4), and the results further confirms that the coke formation and Ni particle growth have been strongly suppressed on [email protected]2O3-IE surrounded catalyst. In general, for Ni-based catalysts for methanation, the smaller Ni nanoparticles, the higher activity and less coke, but along with worse stability due to the sintering of high-active nanoparticles (33). In the present [email protected]2O3-IE surrounded catalyst, the physical isolation of Ni nanoparticles by Al2O3 shell not only offers a large surface area for the high dispersion of active metal but also exerts a spatial restriction on these nickel nanoparticles, hampering their sintering and coke formation in the long-term employment even under harsher conditions (12), displaying the advanced structural design and much superior catalytic performances.
[email protected]2-IE surrounded catalysts with inverse structure (for catalytic transfer hydrogenation) prepared by IEIL method
Catalytic transfer hydrogenation opens up a new dimension in reduction of organic compounds because of operational safety and enhanced degree of control in selectivity compared with the conventional molecular hydrogen–based process. Non-NM catalysts as an alternative for precious metals have attracted extensive attention due to their low cost and abundance but suffering from their relatively poor catalytic activity and selectivity (34). In this context, we fabricate an inverse structure with Ni particles surrounded by CeO2 nanoparticles (denoted as [email protected]2-IE; see Fig. 5, B and C, and fig. S5 in section S5) via IEIL strategy, and the schematic diagram of the synthesis is displayed in Fig. 5A. The CeO2 support is expected to improve the catalytic activity/selectivity via participating in the reaction process (19). For comparison, Ni/CeO2-IM supported catalyst is prepared by the traditional impregnation method.
For the transfer hydrogenation of nitrobenzene with hydrazine to aniline, the [email protected]2-IE surrounded catalyst (Ce/Ni molar ratio of 0.02, Fig. 5, B and C) shows complete conversion of nitrobenzene and 100% selectivity to aniline at 60°C for 1 hour, while Ni/CeO2-IM displays a poor activity (nitrobenzene conversion of 38%) with an aniline selectivity of 76%, eventually only giving an aniline yield of 29% under the same reaction conditions (Fig. 5D). The other products detected by gas chromatography (GC) are nitrosobenzene (NSB) with a selectivity of 6% and azoxybenzene (AOB) with a selectivity of 18%. In addition, the TOF value of [email protected]2-IE (541.6 hour−1) is 5.5 times larger than that of Ni/CeO2-IM (97.9 hour−1), although the later has higher surface Ni concentration (0.038 mmol g−1 versus 0.049 mmol g−1) (Fig. 5E), indicating that Ni particles do not serve as the sole active site. The further investigation discovers a positive relationship between the TOF value and Ce3+/(Ce3+ + Ce4+) ratio (see Fig. 5E, fig. S6A, and section S6). The larger Ce3+/(Ce3+ + Ce4+) ratio suggests more oxygen vacancies (Ov) in CeO2 (XPS results, fig. S6, B to E). Benefiting from the special IEIL method, the obtained mixed metal oxide after calcination usually processes mutually changed interface, which enhances the interaction between Ni2+ and CeO2, resulting in the incorporation of more Ni2+ ions into the CeO2 lattice. The lattice distortion and the enhancement of oxygen mobility benefit the formation of more oxygen vacancies. In contrast, the interfacial interaction on the traditional supported catalyst prepared by impregnation is generally weak. Therefore, the reduced [email protected]2-IE surrounded catalyst processes larger Ce3+/(Ce3+ + Ce4+) ratio and more oxygen vacancies than those of Ni/CeO2-IM supported catalyst (Fig. 5E and fig. S6).
On the basis of the catalytic performances and XPS results, the Ni-CeO2 interfacial perimeter sites may serve as synergistic sites in nitrobenzene reduction reaction. Since [email protected]2-IE has higher vacancy concentration (2.34 times more than those of Ni/CeO2-IM), the oxygen vacancies serve as the adsorption sites for the highly preferential adsorption of nitrobenzene (35, 36), while Ni serves as the active site for N2H4 dissociation and produces the active hydrogen species, which easily spill over to the neighboring interface of CeO2 to reduce the adsorbed nitrobenzene. After the complete hydrogenation by the active spilled-over hydrogen, the resulted aniline can be desorbed from CeO2 support, realizing the superior catalytic activity and selectivity of [email protected]2-IE surrounded catalysts. In contrast, when Ni/CeO2-IM with low oxygen vacancy concentration and high surface Ni concentration is used as a catalyst, nitro group of nitrobenzene will be adsorbed on both the surface of Ni nanoparticles and the surface of CeO2 support. N2H4 activation on Ni nanoparticles produces active hydrogen species, allowing the stepwise reduction of nitrobenzene adsorbed on Ni particles to NSB, hydroxylamine intermediate, and the final product aniline. However, in this case, the coadsorption of these two intermediates and subsequent coupling reactions between them into the by-product of AOB will occur readily, leading to the poor aniline selectivity. Furthermore, the [email protected]2-IE surrounded catalyst could be recycled at least six times with initial catalytic activity (see fig. S7 and section S7), revealing its high stability; in sharp contrast, the activity of supported Ni/CeO2-IM catalyst declined from 38 to 15% after six runs, which highlights the strong interaction (stability) between Ni and CeO2 in the [email protected]2-IE surrounded structure derived from ion exchange.
[email protected] surrounded catalysts with gradient structure (for semihydrogenation of phenylacetylene) prepared by IEIL method
Besides core-shell and inverse structure, when Cu2+ is used to exchange with Ni(OH)2 through IEIL method, a gradient alloy structure varied from nickel-rich core to copper-rich surface can be formed (Fig. 6). The as-prepared [email protected] catalyst (see fig. S8 and section S8) exhibits outstanding styrene selectivity (90%) during the semihydrogenation of phenylacetylene (see table S1 for more catalytic data), even better than some supported NM catalyst such as nano-Pd/Al2O3 catalyst (60%) at the complete conversion of phenylacetylene (37).
The formation of different surrounded structure mainly depends on the exchange degree of the metal hydroxide precursor (Ap, metal ions Am+) with support precursor (Bp, metal ions Bn+) and the inherent properties of both metal precursor and oxide support precursor (e.g., reducibility). In general, when the ion exchange degree is little, [email protected] surrounded catalyst with inverse structure can be obtained, while when metal ion Am+ is exchanged with Bn+ in a large degree, core-shell structured [email protected] surrounded catalyst will be achieved (see fig. S9 and section S9). However, once the oxide surrounding layer and the metal core precursor are thermodynamically susceptible to be reduced to the metallic state simultaneously, gradient alloy structure will be obtained ultimately, as in the case of [email protected] The above examples well demonstrate the universality of IEIL strategy, which paves the new way to controllably fabricate a variety of catalytic materials with various surrounded structures in which high activity and stability can be achieved simultaneously, far superior to their traditional supported counterparts, highlighting the importance of this IEIL strategy for the design of advanced catalysts.