Dually confined Ni nanoparticles by room-temperature degradation of AlN for dry reforming of methane

Applied Catalysis B: Environmental 277 (2020) 118921

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Dually confined Ni nanoparticles by room-temperature degradation of AlN for dry reforming of methane

T

Shuqing Lia,1, Yu Fua,1, Wenbo Konga, Bingrong Pana, Changkun Yuana, Fufeng Caia, He Zhua, Jun Zhanga,*, Yuhan Suna,b,** a

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Science, Shanghai, 201210, PR China b ShanghaiTech University, Shanghai, 201210, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Dry reforming of methane Core-shell Strong metal-support interaction Degradation Room temperature

A core-shell catalyst Ni@Al2O3/AlN was prepared in a facile and environmental way and used for dry reforming of methane (DRM). The Ni nanoparticles (NPs) were supported on AlN and encapsulated by Al2O3 overlayers, which were self-assembled via room-temperature degradation of AlN in moist air. Meanwhile, the nickel ions preferentially incorporate into the tetrahedral vacancies of the AlN to form strong metal-support interaction as demonstrated by theory as well as experiment. Based on the dual confinement effects, the stability of Ni@Al2O3/ AlN catalyst was superior to that of the Ni/AlN and Ni/α-Al2O3, exhibiting negligible coke deposition and small growth of Ni NPs after 100 h time on stream (TOS) at high temperatures (973 and 1073 K). These results demonstrated that this green synthetic strategy offered a promising solution for preventing metal NPs migration and coalescence, especially for the high-temperature catalytic reactions.

1. Introduction Facing with increasing global warming in industrial societies, DRM process is effective in converting two greenhouse gases (CH4 and CO2) into syngas with a H2/CO molar ratio close to unity, which can be used to tune the feedstock ratio for off-stream requirements, such as FischerTropsch and methanol synthesis [1–4]. However, its implementation is mainly at the laboratory research stage, because industrial catalysts toward stable operation remain challenging. Nickel-based catalysts have been widely studied for industrial application scale-up of the DRM process due to their low cost and high activity. Its widespread application, however, is restricted by deactivation, catalyst fracture and reactor blockage caused by the growth of Ni NPs and coke deposition of the catalyst [5–9]. Much effort has been focused on solving those problems by alloying Ni with other metals [9–12], using functional supports or promoters [13–18], and embedding Ni in inorganic cavities [19–21]. Of particular interest is the coreshell catalyst because it exhibits superior stability for the DRM process by providing a physical barrier for the growth of carbon and a confinement effect on the Ni NPs, e.g., Ni@Al2O3 [22], Ni@SiO2 [23] and

Ni-SiO2@CeO2 [24]. Generally, the preparation methods of core-shell catalysts mainly include molecular layer deposition, microemulsion and ammonia evaporation [25–28], as well as hydrothermal synthesis [29]. Those methods can easily control the thickness and pore size of the shell. It is especially interesting that convenient and green synthetic strategies get more and more attentions to avoid complex preparation process and toxic ingredient [30]. For instance, Xu et al. [31] prepared Pt@CeO2 nanocomposite through a template-free hydrothermal approach. Xiao et al. [32] reported the solvent-free synthesis of core-shell Zn/ZSM-5@silicalite-1 catalyst. Nonetheless, the preparation of coreshell catalysts at room temperature has seldom been reported. Herein, we develop a simple and environmentally friendly method for the first time to implement the self-assembly growth of Al2O3 overlayers onto the Ni/AlN surface via room-temperature degradation of AlN and the resulting core-shell material is employed as DRM catalyst to test for performance. This method includes three steps: (1) Ni/ AlN catalyst is prepared by impregnation method; (2) it is then degraded at 25 °C and in 70∼85 % relative humidity (RH) moist air for 7 days; (3) Ni@Al2O3/AlN catalyst, where the Ni NPs are encapsulated between the AlN support and Al2O3 overlayers, is finally obtained after

⁎

Corresponding author. Corresponding author at: CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Science, Shanghai, 201210, PR China. E-mail addresses: [email protected] (J. Zhang), [email protected] (Y. Sun). 1 Shared first co-authorship. ⁎⁎

https://doi.org/10.1016/j.apcatb.2020.118921 Received 25 October 2019; Received in revised form 6 March 2020; Accepted 21 March 2020 Available online 24 March 2020 0926-3373/ © 2020 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic illustration of the formation process of Ni@Al2O3/AlN catalyst.

intermediates were taken on a JEOL JSM-6700 F Field Emission SEM setup. The EDX hardware (Oxford Instruments) that the microscope was equipped with was used for the elemental composition analysis of all supports. Ni loadings in samples were determined by inductively coupled plasma spectrometry (ICP-OES) on an IRIS Intrepid II XSP instrument (Thermo Electron Corporation). Transmission electron microscopic (TEM) images of the powder samples were recorded on a Tecnai G2 F20 STwin instrument operated at 200 kV. The samples were prepared via dispersing in ethanol on 400 mesh copper grids precoated within the carbon films. HAADF-STEM was performed on a JEM-2100 F transmission electron microscope with an accelerating voltage of 200 kV equipped with a post-column Gatan imaging filter (GIF-Tri-dium). The acidic properties of catalysts were measured by NH3 temperature-programmed desorption (NH3-TPD). The NH3-TPD analyses of all the samples were done in the temperature range of 673 K with an increment of 283 K/min. After absorbing the NH3 molecule at 373 K for 30 min, the catalyst began to desorb NH3 under Ar gas and signals were monitored by TCD detector. X-ray photoelectron spectra (XPS) analyses were performed on the Thermo Fisher Scientific ESCALAB 250Xispectrometer, using Al-K radiation (1486.6 eV, pass energy 20.0 eV). The base pressure of the instrument was under 9.9 × 10−7 Torr. All samples binding energies were calibrated using C 1s peak at 284.8 eV. The XPS spectra of O 1s, C 1s, Al 2p, N 1s, Ni 2p regions were measured. The reduction behavior of catalysts was characterized by H2Temperature Programmed Reduction (H2-TPR). Respectively, 100 mg catalyst was carried out in a conventional U-shaped quartz reactor and treated at 473 K with 10 % H2/Ar. In the process, the samples were heated from 473 K to 1273 K for 160 min (the temperature was kept at 1273 K for another 1 h), analysed by a thermal conductivity detector (TCD). The hydrogen consumption values were calculated from the integrated peak area using the mathematical software Origin. The nickel dispersion was derived by CO-pulse chemisorption method [39] and CO2-TPD, which were performed on the dynamic chemical adsorption instrument (Micromeritics, AutoChem II 2920). The CO-pulse chemisorption experiment included two steps: (1) 100 mg sample was reduced at 1073 K for 2 h in H2, then cooled to room temperature and flushed with He for 10 min; (2) 10 % CO/He was pulsed every 2 min at 323 K for 20 times [40]. The total metal dispersion of Ni metal was calculated from the following formula, where DM, VS, SF, TW, and MW were metal dispersion (%), volume of active gas chemisorbed (cm3 at STP), stoichiometry factor, total weight of metal (g), and molecular weight of metal (g/mol), respectively [41].

reduction treatment (Fig. 1). During the degradation process, aluminum nitride surface is hydrolysed to amorphous aluminum oxyhydroxide, which subsequently transforms into mixtures of aluminum trihydroxide polymorphs [33,34]. The reaction can proceed in two steps as follows:

AlN+2H2 O→AlOOHamorph + NH3

(1)

AlOOHamorph + H2 O→ Al(OH)3

(2)

Furthermore, aluminum atoms are present in AlN crystal displaying 4-fold coordination and Ni2+ can incorporate into the tetrahedral vacancies of the AlN [35]. Thus, the synergy of strong Ni-AlN interaction and Al2O3 overlayers has dual confinement effects in the Ni@Al2O3/ AlN catalyst. Moreover, AlN has high thermal conductivity and mechanical strength properties, which also benefit for preventing cold point and catalyst cracking, further exhibiting good potential for the industrial application of DRM [36–38]. In the present work, we aim to study the influence of dual confinement effects in Ni@Al2O3/AlN catalyst on the resistance of carbon deposition and the sintering of Ni NPs as well as its consequence in the effect of reactivity in DRM process. The Ni/α-Al2O3 and Ni/AlN catalysts prepared by impregnation are used as reference samples. 2. Experimental 2.1. Synthetic technique The AlN and commercial α-Al2O3 supports were calcinated at 1573 K for 3 h in N2 and air, respectively. All catalysts were prepared by the incipient-wetness impregnation method. The supports were first impregnated with Ni(NO3)2·6H2O diluted in deionized water, and then dried 3 h at 393 K. Then Ni/AlN and Ni/α-Al2O3 catalyst were obtained after calcinated at 1073 K for 1 h in N2 and air, respectively. Ni@Al2O3/ AlN catalyst was prepared by the room-temperature (25 °C) degradation of Ni/AlN catalyst in moist air (70∼85 % RH) for 7 days. In all cases, the theoretical value Ni content was 0.9 wt %. Aluminum nitride (99.5 % metals basis) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) were obtained from Sinopharm Chemical Reagent Co. α-Al2O3 was obtained from Chalco Shandong Co. All reagents were used as received. Deionized water (18.2 MΩ cm) was used in all syntheses. 2.2. Characterization The XRD patterns were conducted using a Rigaku Ultima IV powder diffractometer with a Cu target Kα X-ray source (λ = 0.154 nm). The 2θ angle increased by 8°/min over a range of 10–90 °. The specific surface areas of the catalysts were measured by N2 adsorption-desorption on a Micrometrics TriStar II 3020 analyser. Prior to analysis, the samples were degassed at 473 K for 10 h. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) Surface morphology of the as-synthesized

DNi (%) = 100 ×

Vs×SF × MW TW×224.14

In CO2-TPD experiments, all samples were first treated in He at 1073 K for 1 h. When the temperature cooled to 293 K, samples were exposed to 5% CO2 (30 mL/min, He in balance) for 30 min, purged in 2

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Table 1 Characterization of all catalysts. Sample

SBETa (m2 g−1)

DRb (%)

NPs sizec (nm)

GHSV

Ni/α-Al2O3 Ni/AlN Ni@Al2O3/AlN Ref. [25] Ref. [44]

5.1 5.6 11.8 61.0 30.7

5.0 4.6 3.0 0.2 N/A

14.7 13.6 13.3 N/A 2.6

1520 1520 1520 1440 N/A

a b c d e f g

d

(L/ g h)

TOFini(CH4)e /s−1

Niw f (%)

H2 Consumption (normalized)

81.2 77.3 64.8 79 61.7

0.92 0.89 0.83 18.6 0.21

1.0 0.81 0.70 N/A N/A

g

Specific BET surface area. Ni dispersion by CO-pulse chemisorption of catalysts reduced at 1073 K for 2 h. Ni average NPs size calculated by TEM. TOF experiments at gas hourly space velocity (GHSV). TOFini(CH4) initial TOF value of CH4 at first 30 min of reaction. The value of Ni content in all catalyst determined by ICP-AES. H2 consumption values calculated from the integrated peak area using the mathematical software Origin by H2-TPR.

He for 1 h at 293 K and heated linearly at 10 K/min to 1073 K in 30 mL/ min. CO2 in the effluent was recorded continuously as functions of temperature [42]. Thermogravimetric mass spectrometric (TGA-MS) analyses were performed on a thermogravimetric analyser (STA449F3) instrument under air to determine the deposited coke amount for spent catalyst. The samples were heated at 1023 K for 80 min, with the flow rate kept at 30 cm3 STP min−1. Raman spectra of the spent catalyst was obtained by using a T64000 Jobin Ivon spectrometer. Approximately, 50 mg samples excited using with an argon laser operating at 514.5 nm and a power of 2 mW.

at 1073 K for 2 h. The reactions of all catalysts were performed at atmospheric pressure at 973 and 1073 K for 100 h, respectively. Finally, the stability test of Ni@Al2O3/AlN catalyst was performed at atmospheric pressure at 1073 K for 300 h. The reactant gases had an inlet molar ratio of CH4/CO2/N2 = 1:1:0.5, which were fed into the reactor at a gas hourly space velocity (GHSV) of 50,000 mL/g h To determine conversion and selectivity, the products were analysed by an on-line gas chromatograph (Agilent Technologies 7890A, 60/80 Carboxen 1000 column) with a TCD detector. The conversions of CH4 and CO2 were calculated from the consumed reactants, and the ratio of H2 and CO was determined from their concentrations. The turn over frequency (TOF) of CH4 for the DRM reactions was defined as the number of CH4 molecules converted over each surface metal site per second. The methane TOF was calculated by the methane conversion rate divided by the number of metal sites, which was determined by CO pulse chemisorption. The TOF experiments of different catalysts were conducted at 1073 K.

2.3. Computational Methods All the density-functional theory (DFT) computations were performed using the Dmol3 software package based on the linear combination of atomic orbitals (LCAO) method. Electron-ion interactions were described using the DFT Semi-core Pseudopots (DSPP) pseudopotentials. A double numerical polarized (DNP) basis set was employed to expand the wave functions with an orbital cutoff of 4.8 Å. For the electron-electron exchange and correlation interactions, the functional parametrized by Perdew-Burke-Ernzerhof (PBE), a form of the general gradient approximation (GGA), was used throughout. The vander Waals interaction was described using the DFT-D2 method proposed by Grimme. During the geometry optimizations, all the atoms were allowed to relax. In this work, the Brillouin-zone integrations were conducted using Monkhorst-Pack (MP) grids of special points. A k-point set with a separation of 0.06 Å−1 was used for all the unit cells. The convergence criterion for the electronic self-consistent field (SCF) loop was set to 10-6. The atomic structures were optimized until the residual forces were below 0.002 Ha Å−1. The key quantity to calculate was the defect formation energy:

ECd = ECT (defect q) − ECT (no defect) +

∑ ui ni − q(εV− εF) i

ECT (defect)

3. Results 3.1. Physical and chemical characterizations of the catalysts The specific surface areas of Ni/α-Al2O3 and Ni/AlN catalysts were 5.1 and 5.6 m2 g−1, respectively (Table 1-SBET). After the degradation of Ni/AlN catalyst, the formed Al2O3 overlayers increased the specific surface area of Ni@Al2O3/AlN catalyst from 5.6 to 11.8 m2 g−1. As estimated in TEM (Fig. 5), the Ni NPs average sizes of three catalysts were over 13 nm, as listed in Table 1. This was because Ni NPs were not sufficiently dispersed on the supports with small specific surface area. In addition, the CO pulse chemisorption tests of different catalysts were carried out to measure the Ni dispersion. The Ni dispersion of Ni@Al2O3/AlN catalyst (3.0 %) was lower than that of Ni/AlN catalyst (4.6 %), probably due to the fact that some Ni° active sites covered by Al2O3 overlayers or part of Ni2+ with strong interaction were not fully reduced (Table 1-DR). In addition, the pore size distribution (Fig. S4) indicated that the Al2O3 overlayers built by primary Al2O3 particles possessed interparticle pores with a size of 10.63 nm. The value of Ni content in all catalysts by ICP-AES characterization was listed in Table 1. The results showed that the value of Ni content of Ni@Al2O3/ AlN catalyst was lower than that of the Ni/AlN and Ni/α-Al2O3 catalysts probably due to the fact that the formed Al2O3 overlayers led to a slight mass increment of AlN support. As shown in the XRD diffractograms of the calcinated samples (Fig. S3b), there was a broad peak at 19.1°, suggesting the existence of NiAl2O4 species. From the XRD diffraction spectra in the range of 10∼30°, the diffraction peak of NiAl2O4 at 19.1° was more clear (Fig. S3a). No peak referred to NiO species was observed for different samples due to equipment limitations or spinel formation. As previously reported, the basic sites of supports could enable dissociative adsorption of CO2 on the catalyst [44]. Accordingly, we

(3)

ECT (no defect)

Where and are the total energy of the supercell “C” with and without the defect, (of charge q,) calculated using the same basis set or planewave cuto, k-point grid,etc, to make use of the cancellation of errors. The defect was formed by adding/removing ni atoms of chemical potential ui. εF was the Fermi level and εv was the valence band maximum (VBM) of the supercell without the defect, which were set to zero here [43]. 2.4. Catalytic reforming reactions Catalytic reactions of DRM were carried out in a quartz tube reactor of 1/2 (inside diameter). The mixture of 100 mg catalyst (40–60 mesh) and 900 mg Quartz sand (40–60 mesh) was loaded (bed length about 1.5 cm) and activated in 120 mL/min flow gas mixture of 50 % H2 in N2 3

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Fig. 2. (a, b) Representative HAADF-STEM image in combination with EDXS element maps of Ni@Al2O3/AlN catalyst after reduction.

3.2. HAADF-STEM of reduced Ni@Al2O3/AlN catalyst

compared the acid-base properties of all samples by NH3-TPD and CO2TPD results (Fig. S5). In the NH3-TPD profile of Ni/AlN catalyst, one peak was observed at 773−1023 K, corresponding to the desorption peak of NH3 from strong acid sites [45]. This peak was attributed to the formation of uncoordinated Al atoms, which were caused by oxygen impurities in the AlN crystal [46,47]. For Ni@Al2O3/AlN catalyst, Al2O3 overlayers weakened the NH3 desorption from strong acid sites, while strengthened the NH3 desorption from weak and medium acid sites. In contrast, no NH3 desorption peak was observed in Ni/α-Al2O3 catalyst. On the other hand, CO2-TPD was applied to investigate the basicity of the calcinated sample, exhibiting a negligible amount of desorbed CO2 for all catalysts as shown in Fig. S5b. This result indicated that the interaction between CO2 and α-Al2O3 support was weak and was nearly the same as the interaction between CO2 and AlN support.

To obtain more detailed information about the shell compositions of Ni@Al2O3/AlN catalyst, HAADF-STEM images and EDXS element maps were employed. As shown in Fig. 2, a large number of O and Al atoms distributed on the edge of Ni@Al2O3/AlN catalyst. It could be inferred preliminarily that the crystal structure of the layered substance was γAl2O3. Furthermore, the core-shell structure of Ni@Al2O3/AlN maintained well and the Ni NPs were isolated from each other by Al2O3 shell with a thickness about 7.5∼15.3 nm (Fig. 2a). In a magnified image (Fig. 2b), the individual Ni nanoparticle with a size about 35 nm was successfully covered by Al2O3 overlayers with thickness ranging from 4∼8 nm. This result indicated that the formed Al2O3 overlayers cover the entire surface of the Ni NPs even if the particle size was very large.

4

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Fig. 3. (a) XPS analyse for Ni/α-Al2O3, Ni/AlN and Ni@Al2O3/AlN catalysts calcinated at 1073 K for 1 h, Ni 2p core level spectra. (b) H2-TPR profiles of the three catalysts.

Ni@Al2O3/AlN catalyst. Ni2+ could easily incorporate into the tetrahedral sites of AlN to form strong Ni-AlN interaction in Ni/AlN and Ni@Al2O3/AlN catalyst, in line with XPS results. For the Ni/α-Al2O3 catalyst, a few Ni2+ incorporated into the octahedral sites and most of the Ni2+ existed in the form of NiO species, leading to the weak interaction between Ni and α-Al2O3. It was well known that SMSI was helpful in preventing the sintering of NPs [52]. Thus, Ni/AlN and Ni@Al2O3/AlN catalysts had smaller average Ni NPs sizes than Ni/αAl2O3 catalyst, as shown in Fig. 5. Moreover, we calculated the amount of H2 consumption for each catalyst from the integrated peak area base on the TPR profiles and defined the peak area of H2 consumption of the Ni/α-Al2O3 catalyst as 1.0. The ratio of the peak area of the Ni/AlN catalysts to the Ni/α-Al2O3 catalyst was 0.81 and the ratio of the peak area of the Ni@Al2O3/AlN catalyst to the Ni/α-Al2O3 catalyst was 0.70 (Table 1). This result suggested that the amount of H2 consumption of the Ni/AlN and Ni@Al2O3/AlN catalyst was lower than that of the Ni/α-Al2O3 a catalyst, exhibiting that a small amount of Ni2+ ions were not reduced due to the strong Ni-AlN interaction. Among them, the Ni@Al2O3/AlN catalyst was the most difficult to reduce, because of the additional strong interaction between Ni NPs and formed Al2O3 overlayers. In Fig. S6, the diffraction peaks of Ni° species at 44.3° were not observed upon reduction for Ni/AlN and Ni@Al2O3/AlN catalysts, pointing to the high dispersion of Ni NPs, which was attributed to the strong Ni-AlN interaction. However, a sharp peak at 44.3° was detected for Ni/α-Al2O3 catalyst, suggesting large Ni NPs, which was due to the weak Ni-α-Al2O3 interaction, in agreement with the TPR results. Furthermore, the XPS results of Ni 2p peaks of catalysts reduced at 1073 K demonstrated that the binding energy of Ni° species at around 853.4 eV for Ni/AlN and Ni@Al2O3/AlN catalysts was higher than that for Ni/α-Al2O3 catalyst (BE at 852.5 eV), which confirmed the conclusion on metal-support interaction (Fig. S7).

3.3. XPS of the catalysts To investigate the difference of metal-support interaction of the three catalysts, XPS analyses were carried out. According to the literature, the binding energies (BE) of P1 peaks of Ni 2p were at 854.5 eV, and the BE of P2 peaks of Ni 2p were at 857.0 eV, which could be divided into two subpeaks: 855.8 eV (Ni2+ in octahedral coordination sites) and 858.2 eV (Ni2+ in tetrahedral coordination sites) [48]. In Fig. 3a, the BE at 856.98 eV and 856.18 eV in Ni/AlN catalyst corresponded to Ni2+ in tetrahedral coordination sites (Ni2tet+ ) and octahedral + coordination sites (Ni2oct ), respectively. This result suggested that the BE 2+ of Nioct was lower than that of Ni2tet+, indicating weak metal-support interaction, in line with the previous report [49]. Meanwhile, the proportion of different Ni2+ species were compiled in Table S1. The + proportion of P1 peaks (Ni2+ existed in NiO species), Ni2oct and Ni2tet+ in Ni/α-Al2O3 catalyst was 51.38 %, 24.52 % and 24.10 %, respectively [35,49]. However, the proportion of Ni2tet+ of Ni/AlN catalyst (50.36 %) was higher than that of Ni/α-Al2O3 catalyst, suggesting that the interaction between Ni and AlN was stronger than that between Ni and αAl2O3. Although the proportion of Ni2tet+ in Ni@Al2O3/AlN catalyst was 38.24 %, lower than 50.36 % of the Ni/AlN catalyst, it was still higher than that in Ni/α-Al2O3 catalyst. The main reason was that part of the Ni2+ incorporated into the octahedral vacancies caused by the formation of Al2O3 overlayers with both 4- and 6-fold coordinated Al3+. + Thus, the proportion of Ni2oct (40.98 %) in Ni@Al2O3/AlN catalyst became higher than that in Ni/AlN catalyst (27.05 %), further proving the above conclusion. 4. Reducibility of the catalysts H2-TPR analysis was performed to further study the metal-support interaction of all catalysts (Fig. 3b). Previous studies suggested that the peaks at 573–873, 873–1023 and over 1023 K corresponded to the reduction of NiO, non-stoichiometric spinel and stoichiometric spinel species into metallic nickel, respectively [50]. For Ni/α-Al2O3 catalyst, the reduction peaks of NiO and non-stoichiometric spinel species were observed, and no reduction peak of stoichiometric spinel species was detected, indicating weak interaction between Ni and α-Al2O3. However, for Ni/AlN catalyst, two reduction peaks of stoichiometric spinel species at about 1090 and 1163 K were detected, implying strong interaction between Ni and AlN. The reduction peak of stoichiometric spinel species at 1163 K corresponded to the Ni2+ in tetrahedral coordination sites [51]. In comparison with Ni/AlN catalyst, the proportion of reduction peak area of stoichiometric spinel species at 1163 K decreased while the proportion of reduction peak area of non-soichiometric spinel species at 973 K increased. It was because Al2O3 overlayers formed non-stoichiometric NiAl2O4 species with NiO species in

4.1. DFT studies For the Ni/AlN catalysts, the Ni-AlN interaction was stronger than that between Ni and α-Al2O3 based on the H2-TPR and XPS results, which benefited for the inhibition of the growth of Ni NPs in the DRM reaction. According to the previous reports, AlN exhibited 4-fold coordinated Al3+ and Ni2+ could easily incorporate into the tetrahedral sites [35,49]. To further explain the migration of Ni ions at the interface between Ni and AlN, α-Al2O3 and γ-Al2O3, DFT calculation was performed. The defect formation energies formed by incorporating Ni ions (Edefect) were calculated for α-Al2O3, AlN and γ-Al2O3 on supercell models, as presented in Fig. 4. The results showed that the Edefect on the Ni/AlN catalyst (-5.4 eV) was lower than that on the Ni/α-Al2O3 catalyst (7.32 eV), pointing to the fact that Ni ions were easier to 5

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Fig. 4. (a): AlN, (b): γ-Al2O3 and (c): α-Al2O3 supercell used for the calculations.

displayed two weak and asymmetrical bands centered at about 1347 and 1590 cm−1, which were associated with the defect and graphite modes, e.g., D band and G band on Ni/α-Al2O3 and Ni/AlN catalysts, respectively. The D band peak was related to the structural imperfections of graphite, and the G band peak was attributed to in-plane carbon-carbon stretching vibrations of graphite overlayers [53]. However, no D or G band peaks were detected on Ni@Al2O3/AlN catalyst, in further agreement with the result of TG.

incorporate into the tetrahedral vacancies of the AlN than the octahedral vacancies of the α-Al2O3. Thus, the Ni/AlN catalyst exhibited a strong Ni-AlN interaction resulting in a chemical confinement for Ni. On the other hand, the Edefect on the Ni/γ-Al2O3 catalyst (1.69 eV) was between that on the Ni/α-Al2O3 catalyst and the Ni/AlN catalyst, suggesting that the Ni-γ-Al2O3 interaction was weaker than the Ni-AlN interaction and stronger than the Ni-α-Al2O3 interaction. This result further proved that part of Ni ions can incorporate into the tetrahedral vacancies at the interface between NiO and the formed Al2O3 overlayers in the Ni@Al2O3/AlN catalyst, in agreement with the as mentioned additional strong interaction between Ni NPs and formed Al2O3 overlayers.

4.3. Catalytic activity and test at high temperature The thermostability tests of catalysts were performed at 1073 K with a molar ratio of CH4/CO2 = 1:1, as shown in Fig. 5d. Ni/AlN and Ni/αAl2O3 catalysts had a similar initial activity, while gradually decreased with TOS. The conversions of CH4 and CO2 were 84.5 % and 90.8 %, reduced by 4.7 % and 4 % for Ni/AlN catalyst, respectively. For Ni/αAl2O3 catalyst, the CH4 and CO2 conversions decreased from 86.9 % and 92.9 % to 80.7 % and 87.4 %, respectively. The initial CH4 conversion of Ni@Al2O3/AlN catalyst was the lowest among the three catalysts because a part of Ni covered by Al2O3 overlayers was difficult to be reduced. The CH4 and CO2 conversions of Ni@Al2O3/AlN catalyst increased from 42.6 % to 69.1 % and 47.1 % to 76.2 % respectively and eventually remained stable with TOS. This was attributed to an increase in the accessible Ni NPs surface area, caused by the expansion of the alumina pores, reduction of NiO under reaction conditions, or both. The expansion of the alumina pores likely resulted from a permanent restructuring of Al2O3 overlayers [26]. To further investigate the stability, we tested the Ni@Al2O3/AlN catalyst at 1073 K and 50,000 mL/g h for 300 h. As illustrated in Fig. 4f, the conversions of CH4 and CO2 were also increased at first and then remained stable with TOS, with no deactivation observed. Considering the dissociative adsorption of methane as rate-determining step for the DRM reaction, the initial CH4 conversion was used to calculate the TOF value. As shown in Table 1, all the catalysts exhibited very close TOFs between 64.8 and 81.2 s−1. The TOFCH4 of these three catalysts at 1073 K was almost the same as the reports by Li et al. [25] and Han et al. [44] because there was a little distinction for the TOFCH4 of catalysts with different metal dispersion under the same reaction conditions [4]. In order to study the reason for the different stability of three

4.2. Catalytic activity and test at low temperature To clearly study the coking resistance of three samples, we evaluated them with a molar ratio of CH4/CO2 = 1:1 at 973 K, under which coking is thermodynamically favorable. Particularly, Ni@Al2O3/AlN catalyst, which needed a strong reducing atmosphere owing to the dual confinement, was activated in a 100 mL/min flow gas mixture of 50 % CH4 in CO2 at 1073 K until reaching a steady state. As shown in Fig. 5a, the CH4 conversions of Ni/α-Al2O3 and Ni/AlN catalysts declined from 53.6 % to 39.4 % and from 63.5 % to 49.1 % after 100 h TOS, respectively. The initial CH4 conversion of Ni@Al2O3/AlN catalyst was 31.4 %, slightly lower than other samples because Al2O3 overlayers covered part of the Ni NPs. Nevertheless, the CH4 conversion of Ni@Al2O3/AlN catalyst only decreased by 3.3 % after 100 h TOS, exhibiting much higher stability. The TG-MS analyses of the spent catalysts after 100 h TOS were performed under air to determine the amount of carbon deposited, as illustrated in Fig. 5b. The results suggested that the amounts of coke deposited on Ni/AlN, Ni/α-Al2O3 and Ni@Al2O3/AlN catalysts were 7.66 wt%, 2.43 wt% and 0, respectively. As reported, the slight rise of TG signal caused by the oxidization of AlN surface at 873 k would affect the calculation of coke deposition. Therefore, the CO2 (m/e 44) mass spectrum signal was used to further confirm the amount of deposited carbon. Clearly, the CO2 mass spectrum signal was not detected on Ni@Al2O3/AlN catalyst and displaying the highest coke resistance. Furthermore, the carbon species of the spent catalysts were confirmed by Raman spectroscopy. In Fig. 5c, the Raman patterns 6

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Fig. 5. (a) CH4 and CO2 conversions for dry reforming of methane with CO2 at 973 K overall catalysts, (b) TG-MS profiles at 973 K, (c) Raman spectra of different spent catalysts at 973 K. (d) CH4 and CO2 conversions for dry reforming of methane with CO2 at 1073 K overall catalysts, (e) XRD profiles of different spent catalysts at 1073 K, (f) stability tests of Ni@Al2O3/AlN catalysts at 1073 K.

there was a large amount of carbon deposition on Ni/AlN catalyst, the sintering of Ni NPs was smaller than that of Ni/α-Al2O3 catalyst because of the strong Ni-AlN interaction. Finally, as confirmed by XRD and TEM, the formation of Al2O3 overlayers could significantly increase the stability of the Ni@Al2O3/AlN catalyst.

catalysts, the XRD and TEM characterizations of spent catalysts were firstly studied. As displayed in Fig. 5e, the peak of the Ni° species at 44.3° was detected on spent Ni/AlN or Ni/α-Al2O3 catalysts, and there was no peak of Ni° species on spent Ni@Al2O3/AlN catalyst after 100 h TOS, suggesting that the sintering of Ni NPs on Ni/AlN and Ni/α-Al2O3 catalysts were more severe than that on Ni@Al2O3/AlN catalyst. Secondly, we found that the average Ni NPs sizes of Ni@Al2O3/AlN, Ni/ AlN and Ni/α-Al2O3 catalysts increased from 13.3–16.9 nm, 13.6–24.3 nm, and 14.7–29.4 nm after a 100-h run at 1073 K, respectively as shown in Fig. 6. Among them, there was the smallest increment in the size of Ni NPs on Ni@Al2O3/AlN catalyst, exhibiting the highest anti-sintering performance, in agreement with the above XRD results. A possible explanation for this was that Al2O3 overlayers effectively prevented the Ni NPs migration and coalescence while Oswald ripening was not completely avoided [54]. For Ni/AlN and Ni/α-Al2O3 catalysts, the Ni NPs migration and coalescence, and Oswald ripening together resulted in the substantial growth of Ni NPs. In addition, coke nanotubes were observed on the surface of the spent Ni/AlN and Ni/αAl2O3 catalysts from the TEM pictures (Fig. 6d, h), suggesting that large uncoated Ni NPs were more likely to cause carbon deposition. Although

5. Discussions AlN support with high thermal conductivity could avoid the formation of cold spots in the monolith or catalyst bed during the DRM reaction. Furthermore, AlN exhibited 4-fold coordinated Al3+ and Ni2+ could easily incorporate into the tetrahedral sites [35,49]. In this study, the defect formation energies formed by incorporating Ni ions at the interface between Ni and AlN, α-Al2O3 and γ-Al2O3, were calculated by DFT to verify the above conclusion. Based on the theory as well as experiments, the Ni-AlN interaction was stronger than that between Ni and α-Al2O3, suppressing the growth of Ni NPs. More importantly, a facile and green method for the preparation of core-shell catalysts was developed. The formed Al2O3 overlayers by degradation of AlN at room temperature successfully encapsulated Ni NPs. This physical 7

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Fig. 6. TEM images of all catalysts after reduction, under H2:N2 = 1:1 at 1073 K for 2 h: (a, b) Ni/α-Al2O3 catalyst, (e, f) Ni/AlN catalyst, (i, j) Ni@Al2O3/AlN catalyst. TEM images of spent catalysts after after 100 h TOS at 1073 k: (c, d) Ni/α-Al2O3 catalyst, (g, h) Ni/AlN catalyst, and (k, l) Ni@Al2O3/AlN catalyst.

Ni@Al2O3/AlN catalyst, the high concentration of oxygen atoms in the surface of AlN caused by the degradation of AlN could destroy the interfacial relaxations to eliminate the Al vacancies [57]. Although the Al2O3 overlayers provided weak and medium acid sites, coke deposition was not detected on the spent Ni@Al2O3/AlN catalyst. This could be attributed to the fact that Al2O3 overlayers efficiently prevent the growth of carbon species by reducing the exposed surface of Ni NPs. As is well known, the ability to form coke required a large and free metallic Ni surface [58,59]. On the other hand, the coverage of Al2O3 overlayers also lowered CH4 and CO2 conversions for Ni@Al2O3/AlN catalyst. Considering the thickness and pore size of the shell directly affected the activity of core-shell catalysts [21,60], the activity could be further improved by the optimization of the temperature, time and humidity of degradation process [33,34].

Fig. 7. Schematic illustration of dual confinement effects of Ni@Al2O3/AlN catalyst.

6. Conclusions

confinement could further efficiently improve the inhibition of the aggregation and migration of Ni NPs. The experimental results showed that Ni@Al2O3/AlN catalyst had superior stability in the DRM reaction owing to the dual confinement effects from strong Ni-AlN interaction and Al2O3 overlayers (Fig. 7). According to the previous study, the Ni NPs size and the acidity of catalysts were very relevant to coke deposition in the DRM reaction [55]. For AlN support, the uncoordinated Al atoms (VAl) could behave as strong electron acceptors, exhibiting Lewis acidity (as shown in the results of NH3-TPD). The formation of VAl mainly resulted from the oxygen impurities that can easily substitute for N atoms, according to the following reaction [56,57]:

Al2O3 → 2AlAl + 3ON + VAl

The room-temperature degradation of AlN in moist air led to a coreshell catalyst Ni@Al2O3/AlN. After the formation of Al2O3 overlayers, there was no coke detected on Ni@Al2O3/AlN catalyst at 973 K for 100 h over DRM because Al2O3 overlayers reduced exposed surface of Ni NPs and provided a barrier for the growth of carbon. Furthermore, Ni2+ could incorporate into the tetrahedral vacancies of the AlN to form strong metal-support interaction, proved by the experiments and DFT calculations. Such dual confinement composed `of a physical barrier and chemical bonding inhibited the growth of Ni NPs on Ni@Al2O3/AlN catalyst. In comparison, substantial coke deposition occurred on the Ni/AlN and Ni/α-Al2O3 catalysts at 973 K, and the size of Ni NPs of those two catalysts significantly increased after a 100 h-run at 1073 K. Therefore, Ni@Al2O3/AlN catalyst was potentially useful for the application of DRM process. This facile and green method could be applied to the design of other metal catalysts for high-temperature catalytic reactions.

(4)

Those VAl could promote coke deposition by enhancing the activation of CH4 [55]. Thus, much coke deposition was detected on spent Ni/ AlN catalyst than on spent Ni/α-Al2O3 catalyst. Interestingly, for 8

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Author contributions

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