Anti-sintering ZrO₂-modified Ni/α-Al₂O₃ catalyst for CO methanation

Abstract

To obviously reduce the sintering behavior of Ni particles in Ni/α-Al₂O₃ catalysts during CO methanation reaction, ZrO₂ was deposited onto the Ni/α-Al₂O₃ catalyst surface by a modified impregnation method. It was observed that the ZrO₂-modified catalyst with an appropriate amount of ZrO₂ showed enhanced catalytic activity due to the increased H₂ uptake and Ni dispersion. During the 103h-lifetime test, the ZrO₂-modified catalyst exhibited higher stability and anti-sintering performance than the unmodified one, because the partial coverage of ZrO₂ particles could effectively prevent Ni particles from sintering during the reaction at high temperatures. The findings obtained in this study should be conducive to the design and development of supported metal catalysts for high temperature reactions with enhanced stability.

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1. Introduction

In the last several decades, CO methanation reaction from syngas (CO + 3H₂ → CH₄ + H₂O) has attracted intensive attention from both academia and industry.^1,2 In this reaction, the syngas can be obtained via gasification reaction of coal or biomass, and Ni/γ-Al₂O₃ catalysts are often employed due to their relatively high activity and low cost.^3,4 However, these catalysts often suffer from Ni sintering during the methanation process because of its strongly exothermic nature^5,6 as well as the presence of water vapor, which is one of the byproducts of methanation and can accelerate the Ni particle sintering.^7,8 In addition, the surface acidity of γ-Al₂O₃ support easily causes coking formation on the catalyst Ni/γ-Al₂O₃ catalysts, and the phase transformation of γ-Al₂O₃ during the high temperature reaction process can lead to the fast deactivation due to collapse of the pore structure and burying of the Ni particles. Therefore, considering the above drawbacks of γ-Al₂O₃ support, some researchers paid their attentions to more stable, acid-free and inert α-Al₂O₃ support.^9,10 Although, partial success has been achieved, the Ni sintering of Ni/α-Al₂O₃ catalysts seems to be more serious due to the low surface area of α-Al₂O₃ support and the weak interaction between NiO species and the support.^9,10 Hence, the anti-sintering property of Ni/α-Al₂O₃ catalyst should be further improved.

Various approaches have been employed to stabilize metal particles, e.g., by adding inorganic oxide via chemical vapor deposition, atomic layer deposition, encapsulating with dendrimer, grafting, or formation of the well-defined cavities or channels, such as core–shell and ordered mesoporous structures.^11–14 However, there are still some drawbacks for these methods, such as the poor control on the shell thickness, the need of special equipment as well as the associated high cost, limiting their wide and large-scale application. ZrO₂ is an inorganic oxide having high thermal stability as well as chemical inertness, and having been reported as the efficient promoter^15–17 or support^18,19 in the catalysts for methanation reaction. However, the sintering-resistance of these catalysts is usually far from ideal except for the ones with ordered mesoporous structure.^17,18 Therefore, improving the anti-sintering property of the catalysts prepared by the conventional methods is in demand.

Recently, several reports showed that the construction of an oxide-on-metal surface structure can enhance both the catalytic activity and stability of metal catalysts dramatically, because they not only combine the function of the metal nanoparticles, but also bring unique collective and synergetic catalytic properties compared to the uncovered metal catalysts.^20–22 Following the previous work on the catalysts with specific oxide-on-metal structures in our group for CO methanation,^14,15 the ZrO₂-modified Ni/α-Al₂O₃ catalysts were prepared by a modified impregnation method in this work. To the best of our knowledge, there is no report in the literature for such route for CO methanation reaction. In addition, in order to obtain a deep insight into the interrelationship among the structure, activity and stability, we investigated the catalytic performances of the catalysts and characterized their structures. It is found that the ZrO₂ nanoparticles can distribute over the surface of the catalyst and even partially cover some Ni particles, which can restrain the growth of the Ni particles and result in enhanced stability in the lifetime test.

2. Experimental

2.1. Catalyst preparation

Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and ethanol were analytical grade (Sinopharm Chemical Reagent Co. Ltd., China). Zirconium n-butoxide (Zr(OBu)₄) (purity 80 wt% in n-butanol) was used without further treatment (Sigma-Aldrich). The α-Al₂O₃ was obtained by calcination of the commercial γ-Al₂O₃ (purity > 95% GongYiHuaYu Alumina Co. Ltd., China, 300 m² g⁻¹) at 1200 °C in air for 5 h.

2.1.1. Preparation of NiO/α-Al₂O₃ catalyst. The NiO/α-Al₂O₃ catalyst was prepared by the wet impregnation method according to our previous report.¹⁴ Typically, 0.39 g Ni(NO₃)₂·6H₂O was dissolved in 20 mL deionized water, followed with addition of 1.00 g of the α-Al₂O₃ powder, then the slurry was kept under vigorous stirring at room temperature overnight. After evaporation of the liquid at 80 °C, the sample was calcined at 400 °C for 2 h in air. The obtained NiO/α-Al₂O₃ catalyst was denoted as 10NA with a NiO loading of 10 wt%.

2.1.2. Preparation of ZrO₂-modified NiO/α-Al₂O₃ catalyst. ZrO₂-modified NiO/α-Al₂O₃ catalyst was prepared by the modified impregnation method after the above procedure. Typically, for each cycle, 1.00 g 10NA was immersed in a Zr(OBu)₄ ethanol solution (5 mL 0.05 M) for 1 min in a sand core funnel to allow adsorption of Zr(OBu)₄ onto the 10NA. After filtered and washed twice with anhydrous ethanol, 5 mL 0.1 M ethanol aqueous solution was added sequentially within 1 min to initiate the reaction of the pre-adsorbed Zr(OBu)₄ with H₂O to form the desired ZrO₂ precursor. The substrates were then dried at 100 °C in air. Such a modification cycle (Scheme 1) could be repeated for several times to achieve the desired amount of ZrO₂. The obtained samples were finally calcined at 400 °C for 2 h and denoted as 10NA@ZrO₂-x (x = 5, 10 and 15), where x represents the cycle times. In addition, the bulk ZrO₂ was obtained through the calcination of the hydrolyzed Zr(OBu)₄ under the same calcination condition as 10NA.

Scheme 1 Illustration of the one impregnation cycle of the ZrO₂-modified Ni/Al₂O₃ catalyst.

2.2. Catalysts characterization

N₂ adsorption was measured at −196 °C (Quantachrome surface area & pore size analyzer NOVA 3200e) and degassed at 200 °C for 4 h under vacuum. The specific surface area was determined according to the Brunauer–Emmett–Teller (BET) method. PANalytical X'Pert PRO MPD was used to record the X-ray diffraction (XRD) patterns with the Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Quantachrome chemBET pulsar TPR/TPD was used to record both H₂ temperature-programmed reduction (H₂-TPR) and H₂ temperature-programmed desorption (H₂-TPD) profiles. Similar with our previous report,¹³ 0.05 g sample was used in the H₂-TPR measurement. After being heated from room temperature to 200 °C at 10 °C min⁻¹ and maintained for 1 h in a He flow, the sample was cooled to room temperature and followed by heating to 1000 °C at 10 °C min⁻¹ in the 10 vol% H₂/Ar flow (30 mL min⁻¹). In the H₂-TPD measurement, 0.2 g catalyst was used and reduced in situ in the H₂/Ar flow at 650 °C for 2 h, then the sample was cooled to room temperature and saturated with H₂ for 1 h. After being flushing with Ar for 1 h, the sample was heated to 600 °C at 10 °C min⁻¹ in the Ar flow (30 mL min⁻¹). The signal was detected using a thermal conductivity detector (TCD). The dispersion of Ni was calculated using the method described in our previous work.¹³ The morphology of the samples was observed by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) (JSM-6700F, JEOL, Japan) and transmission electron microscopy (TEM) (JEM-2010F, JEOL, Japan) under a working voltage of 200 kV. The surface chemical composition of the reduced catalysts was measured using X-ray photoelectron spectroscopy (XPS) test conducted on a VG ESCALAB 250 spectrometer (Thermo Electron, U.K.) with a non-monochromatized Al Kα X-ray source (1486 eV). In addition, the exact composition of the catalysts was determined by using a Thermo Scientific iCAP 6300 inductively coupled plasma atomic emission spectrometry (ICP-AES). Moreover, thermogravimetric (TG) analysis was conducted on a Seiko Instruments EXSTAR TG/DTA 6300 in the temperature range of room temperature to 1000 °C (10 °C min⁻¹) in air.

2.3. Catalytic measurement

The catalysts (20–40 mesh) were reduced at 650 °C in pure H₂ (100 mL min⁻¹) for 2 h and then cooled to the starting reaction temperature in H₂ in a fixed-bed quartz tubular reactor (10 mm I.D.) at 0.1 MPa. Then the mixed H₂ and CO as well as N₂ (as an internal standard) (H₂/CO/N₂ = 3/1/1, molar ratio) were introduced into the reactor at a weight hourly space velocity (WHSV) of 30 000 mL g⁻¹ h⁻¹. The inlet and outlet gases were analyzed on line by Micro GC 3000A (Agilent Technologies) with a TCD detector. The lifetime test of CO methanation was performed at 500 °C, 0.1 MPa, 60 000 mL g⁻¹ h⁻¹. Hydrothermal treatment for catalysts aging was also carried out in a fixed bed quartz tube reactor under 0.1 MPa.¹⁷ Prior to the treatment, the catalyst (10NA and 10NA@ZrO₂-5) was reduced at 650 °C in pure H₂ for 1 h and then subjected to 90 vol% H₂O/H₂ at 650 °C for 8 h. After being cooled to room temperature in pure H₂, the obtained samples were denoted as 10NA-HT and 10NA@ZrO₂-5-HT, respectively. The CO conversion, CH₄ selectivity and yield are defined as follows:¹³


here, X is the conversion of CO, S is the selectivity of CH₄, Y is the yield of CH₄, F_i,in and F_i,out are the volume flow rates of species i (i = CO or CH₄) at the inlet and outlet.

3. Results and discussion

3.1. Characterization of the catalysts

All the calcined samples show the similar N₂ adsorption–desorption isotherms with type IV isotherms and H4 hysteresis loops (Fig. 1A), suggesting the textural structure still remains after the incorporation of ZrO₂ into 10NA. After reduction at 650 °C for 1 h, there is no obvious change in their isotherms (Fig. 1B), and the hysteresis loops of the samples locate at P/P₀ = 0.75–1.0, indicating the pores are from the aggradation of nanoparticles.⁹ Table 1 lists the surface areas of the calcined and reduced samples. Overall, the reduced samples have the lower surface areas than the calcined ones, and the surface areas of 10NA@ZrO₂-x samples are slightly increased with the increase of the ZrO₂ amount, because the small ZrO₂ nanoparticles can make the surface of the catalyst much coarser. In addition, the exact compositions of the above catalysts are determined by ICP-AES and listed in Table 1. The ZrO₂ loading varies from 3.10 to 12.05 wt%, and the real Ni loading of 10NA@ZrO₂ is slightly lower than 10 wt% due to the addition of ZrO₂ to 10NA.

Fig. 1 N₂ adsorption isotherms of the calcined (A) and reduced (B) samples: (a) 10NA, (b) 10NA@ZrO₂-5, (c) 10NA@ZrO₂-10, and (d) 10NA@ZrO₂-15 (for clarity, the isotherm of 10NA, 10NA@ZrO₂-5, 10NA@ZrO₂-10 and 10NA@ZrO₂-15 was vertically shifted for 80, 55, 35 and 0 cm³ g⁻¹, respectively).

Table 1 Physical and chemical properties of the samples

Catalysts	SBET^a (m^2 g^−1)		NiO content^b (wt%)	ZrO2 content^b (wt%)	Ni particle size (nm)		H2 uptake (μmol g^−1)	D^e (%)
	Calcined	Reduced			XRD^c	TEM^d
a Surface area of the sample derived from BET equation.b The exact composition of the calcined catalysts was determined by ICP-AES.c Particle size estimated from the XRD diffraction peak (2θ = 44.6) using the Debye–Scherrer equation.d Average particle size estimated from the TEM images.e Ni dispersion calculated from the H2-TPR and H2-TPD results.
10NA	25	12	—	—	10.3	10.3	23.9	3.6
10NA@ZrO2-5	27	17	9.53	3.10	8.8	7.2	44.0	6.5
10NA@ZrO2-10	29	19	9.10	7.21	9.3	7.5	33.1	4.9
10NA@ZrO2-15	30	21	8.35	12.05	9.2	7.0	23.0	3.4

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Fig. 2 shows the morphology and XRD patterns of the 650 °C-reduced catalysts. In 10NA, the Ni nanoparticles are highly dispersed over the support with the average Ni size of 10.3 nm (Fig. 2a), which is consistent with the result of XRD in Table 1. ZrO₂ nanocrystallites can be easily identified in the TEM images due to the obvious contrast difference between α-Al₂O₃ and ZrO₂. It is seen that some ZrO₂ particles are distributed on the surface of the catalysts (Fig. 2b and c), and even cover the dominating surface at higher ZrO₂ loadings (Fig. 2d). The coverage of ZrO₂ species are expected to improve the interaction between NiO and the support, and restrain the growth of the Ni particles during the reduction and catalytic reaction at high temperatures. Actually, the average Ni particle sizes are about 7.2, 7.5 and 7.0 nm in 10NA@ZrO₂-5 (Fig. 2b), 10NA@ZrO₂-10 (Fig. 2c) and 10NA@ZrO₂-15 (Fig. 2d), respectively, which is smaller in size compared to that of 10NA. Fig. 2e shows the HRTEM image of the reduced 10NA@ZrO₂-15, which reveals more information on the distribution of ZrO₂ particles. The observed lattice spacing of ca. 0.20 nm corresponds to the Ni (111) plane,¹³ and the ones at ca. 0.26 and 0.30 nm to the α-Al₂O₃ (104) plane and the tetragonal ZrO₂ (t-ZrO₂) (011) plane, respectively. Clearly, the ZrO₂ species covers part of the catalyst surface, which should include some Ni particles, while the rest Ni particles are distributed among the ZrO₂ particles. Furthermore, the results of the elemental mappings and EDS of the reduced 10NA@ZrO₂-15 confirm that both Zr and Ni elements are well dispersed across the whole Al₂O₃ support (Fig. 3). In Fig. 2f, the observed peaks at 25.6, 35.2, 43.4 and 57.9° correspond to (012), (104), (113), and (116) planes of α-Al₂O₃ (JCPDS 01-082-1468), while those at 44.5, 51.8 and 76.6° to (111), (200) and (220) planes of metallic Ni (JCPDS 01-070-1849). After the ZrO₂ addition, new diffraction peaks at 30.3, 50.4 and 60.2° are observed in the XRD patterns, which correspond to (011), (112) and (121) planes of t-ZrO₂ (JCPDS 00-050-1089), while the ones at 28.2 and 31.5° attributing to (−111) and (111) planes of monoclinic ZrO₂ (m-ZrO₂). Clearly, the mixed crystal phases of ZrO₂ exist in the reduced catalysts. Furthermore, the Ni particle sizes are calculated using the Debye–Scherrer equation and listed in Table 1. Overall, the Ni particle sizes of the ZrO₂-modified catalysts are smaller than that of 10NA, and the Ni particle size calculated through XRD is consistent with that estimated from TEM, which confirms that the steric hindrance of ZrO₂ can restrict the sintering of the Ni particles during the reduction process at high temperature.

Fig. 2 TEM images of the reduced samples: (a) 10NA, (b) 10NA@ZrO₂-5, (c) 10NA@ZrO₂-10, and (d and e) 10NA@ZrO₂-15; and XRD patterns of the reduced catalysts (f).

Fig. 3 SEM image of the reduced 10NA@ZrO₂-15 (a), elemental mapping images of O (b), Al (c), Ni (d), and Zr (e); energy-dispersive X-ray spectroscopy (EDS) analysis of this sample (f).

Fig. 4A presents the H₂-TPR profiles of the catalysts as well as the bulk ZrO₂. It should be pointed out that there is no any reduction peak for ZrO₂, and hence all the hydrogen consuming peaks of the catalysts are attributed to the NiO reduction. There are three reduction peaks at about 305, 362 and 524 °C in the profile of 10NA, which corresponds to NiO with weak (305 and 362 °C) or middle (524 °C) interactions with the support.³ The reduction peaks at low temperature are disappeared after the ZrO₂ addition, and shifted to high temperature range (475 and 616 °C), because the NiO species is covered partially by ZrO₂ particles, which significantly increases the difficulty of the reduction of NiO species.³

Fig. 4 H₂-TPR profiles (A) and H₂-TPD profiles (B) of the catalysts: (a) 10NA, (b) 10NA@ZrO₂-5, (c) 10NA@ZrO₂-10, (d) 10NA@ZrO₂-15 and (e) ZrO₂.

Fig. 4B shows the H₂-TPD profiles of all the catalysts and the bulk ZrO₂. There is a negligible H₂ adsorption peak for ZrO₂, which is quite similar to the results in the literature,¹⁵ and thus all the hydrogen desorption peaks in the H₂-TPD profiles of the catalysts can be contributed to the Ni species. The H₂-TPD profile of 10NA shows two main H₂ desorption peaks at around 153 and 296 °C. After the ZrO₂ addition, there is an obvious increment in the integrated area of the low-temperature peak (153 °C) as compared with that of 10NA; meanwhile, the peak at 296 °C is disappeared, indicating the presence of more highly-dispersed Ni atoms.³ However, the intensity of the H₂ desorption peak decreases gradually with the increase of the ZrO₂ addition due to the coverage of some exposed Ni particles by excess ZrO₂. The hydrogen uptakes and Ni dispersion of the catalysts are listed in Table 1. The modification of ZrO₂ is beneficial for enhancing the hydrogen uptakes and Ni dispersion. Among these catalysts, 10NA@ZrO₂-5 has the highest total H₂ uptake of 44.0 μmol g⁻¹ and the Ni dispersion of 6.5%. In addition, we notice that the Ni dispersion of 10NA@ZrO₂-5 is almost 2 times higher than that of 10NA, which is contradictory to the fact that their Ni particle size difference is considerable small (1.5 nm). This may be understood from the results of H₂-TPR, in detail, there is a stronger interaction between Ni species and support in 10NA@ZrO₂-5 than that in 10NA (Fig. 4A). Thus the sintering degree of Ni particle in the former should be lower during the 650 °C-reduction process, resulting in the formation of more exposed surface active Ni atoms with high density of surface defects,¹³ and much improved Ni dispersion. However, for 10NA@ZrO₂-10 and 10NA@ZrO₂-15, the excess ZrO₂ has the negative effect on H₂ uptake because of the severe coverage of Ni species. In all, only the proper amount of ZrO₂ can lead to the improved dispersion of Ni particle in this work.

The XPS spectra of the reduced 10NA and 10NA@ZrO₂-5 catalysts are shown in Fig. 5. The reduced 10NA catalyst shows the metallic Ni (Ni⁰) peak at around 852.3 eV, the peaks at 855.8 and 861.5 eV are assigned to the Ni oxide states (Ni²⁺) because catalysts were oxidized in air during the sample transfer and preparation (Fig. 5a).¹⁶ After addition of ZrO₂, there is no obvious change of the Ni⁰ binding energy, indicating ZrO₂ species is a structure promoter rather than the electron promoter in the 10NA@ZrO₂-x catalysts for CO methanation. The similar Ni⁰/(Ni⁰ + Ni²⁺) of the two catalysts indicates the similar reducibility of Ni spices in the different catalysts. The O 1s spectra are displayed in Fig. 5b. There is only one peak in the O 1s spectra of 10NA, while two kinds of oxygen species can be observed in 10NA@ZrO₂-5. The low binding energy peak was assigned to the surface lattice oxygen (O_I), and the other is the chemisorbed oxygen (O_II)¹⁸ which is the typical characteristic of ZrO₂. The adsorbed oxygen of 10NA@ZrO₂-5 can supply more oxygen vacancies which are favorable for eliminating deposited carbon and improving the stability of the catalyst.

Fig. 5 XPS spectra of the reduced catalysts: (a) Ni 2p and (b) O 1s.

3.2. Catalytic performances of the catalysts

The CO methanation reaction was carried out in the temperature range of 260–440 °C at 0.1 MPa, 30 000 mL g⁻¹ h⁻¹, and the results are shown in Fig. 6a–c. For 10NA, the CO conversion increases with the increase of the reaction temperature and reaches the maximum of 87% at 440 °C; at the same time, the CH₄ selectivity drops with the rise of the temperature due to the side reactions such as water-gas shift and inversed methane CO₂ reforming reaction.²³ It can be seen that the addition of ZrO₂ considerably improves the catalytic activities, and 10NA@ZrO₂-5 shows the best catalytic performance, whose CO conversion and CH₄ yield can reach 94% and 74% respectively at 400 °C. However, excess ZrO₂ is adverse for the catalytic performance, and the activities of 10NA@ZrO₂-x (x = 10 and 15) decrease with the further increase of the ZrO₂ loading. The order of the activity of the different catalysts is consistent with that of the H₂ uptake, which is similar to the literature results.^3,24 It should be pointed out that the order of CH₄ selectivity of the different catalysts is opposite with their order of CO conversion, indicating the side reactions can also be significantly enhanced in the presence of the more active catalysts. In all, only the proper amount of ZrO₂ can remarkably enhance the CO methanation, and the excess ZrO₂ has the adverse effect on it due to the coverage of the active sites.

Fig. 6 Catalytic properties of the catalysts: (a) CO conversion, (b) CH₄ selectivity, and (c) CH₄ yield; and lifetime test of 10NA and 10NA@ZrO₂-5 catalysts: (d) CO conversion, (e) CH₄ selectivity, and (f) CH₄ yield.

In order to evaluate the lifetime of the catalyst in a short time in laboratory, a 103h-lifetime test of 10NA@ZrO₂-5 was carried out at high temperature and WHSV (500 °C, 60 000 mL g⁻¹ h⁻¹, 0.1 MPa), and 10NA was also evaluated as a comparison (Fig. 6d–f). During the test, the CO conversion and CH₄ yield over 10NA are decreased by 17.8% and 13.1%, respectively, although the CH₄ selectivity (Fig. 6e) is still comparable. On the contrary, the CO conversion and CH₄ yield over 10NA@ZrO₂-5 show the decrement of 6.7% and 5.5%, respectively. Clearly, 10NA@ZrO₂-5 exhibits a better stability especially at high temperature and WHSV. In Fig. 7, it can be seen that in the spent 10NA (Fig. 7A) and 10NA@ZrO₂-5 (Fig. 7B), the Ni nanoparticle size is 12.2–25.1 and 7.0–12.5 nm, respectively. Compared with the aforementioned TEM images of the reduced catalysts (Fig. 2), the anti-sintering of Ni particle on 10NA@ZrO₂-5 is significant. At the same time, the average Ni particle size of the spent 10NA calculated from XRD pattern is 17.2 nm, while that of the spent 10NA@ZrO₂-5 is 9.5 nm, only slightly larger than that of the reduced catalysts (Table 1), further confirming the anti-sintering property of 10NA@ZrO₂-5. We believe that the anti-sintering performance can be further improved through optimization of the amount of ZrO₂. Moreover, the amount of carbon deposited on the spent catalysts can be measured by TG analysis, and the result is presented in Fig. 7D. The TG curves of the samples exhibit a rise in the temperature range of 200 to 430 °C, which results from the oxidation of the metallic Ni.²⁵ Furthermore, the decrement of mass loss percentage of the spent 10NA catalyst is larger than that of 10NA@ZrO₂-5, indicating a higher resistance of 10NA@ZrO₂-5 to coke formation. The TEM image of the spent 10NA catalyst further confirms the existence of the filamentous carbon. However, the amount of the deposited carbon of the two catalysts is close, suggesting that coke may be not the main factor of deactivation. In short, the sintering of Ni particles leads to the decrease of the activity of 10NA in the lifetime test, and 10NA@ZrO₂-5 exhibits high catalytic activity and stability as well as much improved resistance to Ni sintering.

Fig. 7 TEM images of the spent samples: (A) 10NA, and (B) 10NA@ZrO₂-5; XRD patterns of the spent catalysts (C): (a) 10NA, and (b) 10NA@ZrO₂-5; and TG curves of the spent catalysts in air (D) (inset: TEM image confirming the carbon filament).

3.3. Hydrothermal stability of the catalysts

Recently, researchers have reported that H₂O²⁶ and OH anions (NaOH in water)²⁷ could significantly enhanced the activity and selectivity of Ni based catalysts for C–O bond hydrogenolysis of lignin model compounds and organosolv lignin. Considering water is one of the byproducts in CO methanation and additional steam in the feed gas is an often used strategy to avoid the generation of hot spots in the catalyst bed and to reduce carbon deposition in industry,^9,17,28 the hydrothermal stability of 10NA@ZrO₂-5 was examined, and that of 10NA was also carried out as the reference. Fig. 8 shows the catalytic properties of 10NA@ZrO₂-5-HT and 10NA-HT after the hydrothermal treatment. Compared with the fresh catalysts in Fig. 6, 10NA@ZrO₂-5-HT shows comparable catalytic activity and selectivity; while the activity of 10NA-HT decreases drastically, suggesting that the introduction of proper amount of ZrO₂ is useful to stabilize Ni particles on α-Al₂O₃ support. In addition, the Ni particle sizes of the two hydrothermally treated catalysts were also estimated using XRD analysis (Fig. 9). The Ni particle size grows from 10.3 (Table 1) to 25.3 nm in 10NA-HT, which is even larger than that of the spent 10NA catalyst (17.2 nm); while the change in 10NA@ZrO₂-5-HT is from 8.8 (Table 1) to 12.8 nm, which further indicates the superior anti-sintering property of 10NA@ZrO₂-5. In addition, there is no obvious change of crystallization of ZrO₂ during the hydrothermal treatment.

Fig. 8 Catalytic properties of the catalysts after hydrothermal treatment: (a) CO conversion, (b) CH₄ selectivity, and (c) CH₄ yield.

Fig. 9 XRD patterns of the catalysts after hydrothermal treatment.

3.4. Schematic diagram of the catalysts

Combing and considering all the above results, a schematic diagram illustrating the preparation process and the structure–property relationship of ZrO₂-modified Ni/Al₂O₃ catalyst is depicted and shown in Fig. 10. The calcined 10NA successively adsorbs Zr(OBu)₄ and H₂O during the preparation process, and amorphous ZrO₂ (a-ZrO₂, XRD pattern not shown here) is obtained after the hydrolysis of Zr(OBu)₄ and the calcination (Fig. 10a and b). After reduction, tetragonal and monoclinic phases of ZrO₂ (t-ZrO₂ and m-ZrO₂) will be formed, and the ZrO₂ particles that are interspersed among the nickel species can act as a physical barrier to restrict the growth of the Ni particles and enhance the anti-sintering of the Ni species at high temperatures. However, the excess ZrO₂ may be beneficial to the improved stability, but it has adverse effect on the catalytic activity due to the coverage of some exposed Ni atoms.

Fig. 10 Schematic diagram illustrating the preparation of the ZrO₂-modified Ni/α-Al₂O₃ catalyst: (a) the calcined 10NA, (b) the calcined 10NA@ZrO₂-15, and (c) the reduced 10NA@ZrO₂-15 (note: a, t and m is the abbreviation of amorphous, tetragonal and monoclinic, respectively).

4. Conclusions

ZrO₂-modified Ni/α-Al₂O₃ catalysts were prepared by a modified impregnation method and used for CO methanation to produce synthetic natural gas. Compared to the unmodified catalyst, the 10NA@ZrO₂-5 catalyst shows superior catalytic activity, coinciding with its large H₂ uptake and high Ni dispersion. In a 103h-lifetime test conducted under the conditions at 500 °C, 0.1 MPa and 60 000 mL g⁻¹ h⁻¹, 10NA@ZrO₂-5 displays a high stability and resistance to Ni sintering. Further characterizations indicate that in the preparation process, ZrO₂ particles are formed and interspersed among the nickel particles to restrain the growth of the Ni particles during the reduction and catalytic reaction at high temperatures, resulting in the high stability in the lifetime test. It is expected that this approach for catalyst preparation can be widely applied in the metal-based catalysts to enhance their anti-sintering performance in some high-temperature reactions.

Acknowledgements

The authors gratefully acknowledge the supports from the National High Technology Research and Development Program 863 (No. SS2015AA050502), the Fund of State Key Laboratory of Multiphase complex systems (No. MPCS-2015-A-06), the National Natural Science Foundation of China (No. 21476238), the National Basic Research Program (No. 2014CB744306), the Open Research Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCS-2014-D-03) and “Strategic Priority Research Program” of the Chinese Academy of Sciences (No. XDA07010100 and XDA07010200). Z. Zhong (Zhong_ziyi@ices.a-star.edu.sg) works in Institute of Chemical Engineering in Singapore, and also holds an adjunct associate professor position in the School of Chemical & Biomedical Engineering, Nanyang Technological University (NTU) in Singapore.

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