Fine-tunable Ni@porous silica core–shell nanocatalysts: Synthesis, characterization, and catalytic properties in partial oxidation of methane to syngas

doi:10.1016/j.jcat.2012.01.004

Journal of Catalysis

Volume 288, April 2012, Pages 54-64

https://doi.org/10.1016/j.jcat.2012.01.004 Get rights and content

Abstract

Ni nanoparticles (NPs) of narrow size distribution encapsulated inside meso- and microporous silica were prepared through in situ reduction of NiO NPs coated with silica. By varying preparation parameters, the mean size of Ni NPs can be fine-tuned in the range 6–45 nm. It was found that with variation in core size, microcapsular cavity, and shell porosity, the as-obtained Ni@meso-SiO₂ catalysts for the partial oxidation of methane to synthesis gas are notably different in catalytic activity and durability. The catalyst activity and durability are essentially determined by the size of the Ni cores, and also somewhat by the porosity of SiO₂ shells, as well as the extent of core–shell interaction, which is influenced by the microcapsular cavity structure. The Ni-350@meso-SiO₂ catalyst with Ni NPs of ca. 6 nm and SiO₂ shells with 3–4 nm mesopores is superior in both activity and durability, giving CH₄ conversion of ∼93%, H₂ selectivity of 92–93% (750 °C and GHSV = 72,000 mL g⁻¹ h⁻¹), and TOF_CH4 of 37.9 s⁻¹.

Graphical abstract

Ni nanoparticles with controllable size and a narrow size distribution encapsulated inside meso- and microporous silica were prepared for partial oxidation of methane. The Ni-350@meso-SiO₂ catalyst with Ni particles of ca. 6 nm is superior in both activity and durability at 750 °C and gas hourly space velocity (GHSV) of 72,000 mL h⁻¹ g⁻¹.

Highlights

► Ni nanoparticles (6–45 nm) encapsulated by silica shell of different porosity. ► Catalysts are highly active for partial oxidation of methane to synthesis gas. ► Ni size, shell porosity, and core-shell interaction determine catalyst activity and durability. ► Ni-350@meso-SiO₂ with Ni cores of 6 nm is superior in both activity and durability.

Introduction

Production of synthesis gas (syngas) through the partial oxidation of methane (POM) or catalytic oxy–methane reforming (COMR) is of great interest [1]. Compared with the traditional method of steam reforming, POM has the following advantages: (i) syngas with an H₂/CO ratio of approximately 2 can be obtained, ideal for processes such as methanol [2] and Fischer–Tropsch synthesis [3], and (ii) the POM process is energetically more favorable because it is slightly exothermic. Usually, Ni and noble metals such as Pt and Rh are used as active components for this reaction [1], [4], [5].

Because of the high cost of noble metals, various Ni catalysts have been developed for the POM process [6], [7], [8], [9], [10]. Huszar et al. [11] first reported that Ni supported on alumina was an efficient catalyst for POM, and the yields were close to the equilibrated values. Besides Al₂O₃, other materials such as SiO₂ and MgO were also adopted as support. Choudhary et al. [12], [13] studied NiO deposited on refractory supports precoated with MgO, CaO, SrO, BaO, Sm₂O₃, and Yb₂O₃. Despite low surface area, the Ni catalyst in which the support was precoated with MgO and the as-fabricated catalyst calcined at 1173 K exhibited high CH₄ conversion and syngas selectivity. Wang et al. [14] studied the effect of doping Ce, La, and Ca on Ni/γ-Al₂O₃ and Ni/α-Al₂O₃ catalysts and found that there was enhanced CH₄ conversion and CO selectivity. The ignition temperature was found to decrease over the Ni-based catalysts that were modified by Pd, Pt, and Ru. In these studies, however, the effects of dopant–support interaction and metal particle size were strongly interdependent, making it difficult to differentiate the role of the support from the effect of particle size.

In the reported studies, the catalysts were usually prepared by impregnation followed by calcination/reduction. It is observed that the size distribution of metal clusters varies with the kind of support. Lu et al. reported that variation in Ag particle size on CaCO₃ showed rather different effects on ethylene and propylene epoxidation [15] and that very small Au particles (∼1 nm) supported on mesoporous Ti-containing silicate are the most active for propylene epoxidation [16]. Recently, encapsulation of metal nanoparticles (NPs) with a porous shell such as silica or alumina was found to stabilize the metallic NPs against sintering at high temperatures [17], [18], [19], [20], [21]. The NPs isolated inside inert shells have a relatively uniform environment around the core surface. Joo et al. reported that as a model catalyst, Pt cores enwrapped inside a mesoporous silica shell were thermally stable for ethylene hydrogenation and CO oxidation [20]. It was reported that even after pretreatment in air at 700 °C, the silica-encapsulated Pd catalysts were stable for CO oxidation and acetylene hydrogenation [17], [21]. Compared to a traditional Pd/SiO₂ catalyst, the core–shell structured Pd@SiO₂ counterpart showed higher resistance toward deactivation and could easily be regenerated without sintering of core particles over multiple reaction–regeneration cycles. Li et al. [22] prepared Pd@SiO₂ structures via a two-step procedure, resulting in a nonuniform distribution of Pd size and thick SiO₂ shells. The catalyst, however, was found to be highly active for 4-carboxybenzaldehyde hydrogenation to p-toluic acid.

Takenaka et al. [23], [24] reported that silica-coated Ni catalysts showed high activity and improved stability in the steam reforming of propane and POM. Compared with conventional Ni/Al₂O₃, Ni/MgO, and Ni/SiO₂ catalysts, core–shell structured Ni catalysts show less carbon deposition. It was thought that the strong interaction of the Ni cores with the silica shell prevents Ni particles from sintering as well as hindering carbon deposition. Our previous studies on core–shell type catalysts suggested that, compared to the core–shell structures of close contact, the ones with cavities generated between the core surface and the inner wall of the shell via in situ reduction of oxide cores can serve as a microcapsule-like reactor and enhance surface adsorption and catalytic reactivity [25], [26], [27].

Porous silica not only allows access of reactant molecules to core particles, but also prevents core NPs from aggregating during POM at high temperatures. It is possible for us to design a core–shell type catalyst in which the size of metallic core particles can be fine-tuned (depending on the size of the corresponding oxide precursor) and retain a rather uniform size distribution (Schemes 1A and B). The size (and hence size distribution) of particles can essentially be maintained with time on stream owing to the protection of a stable SiO₂ shell. This allows us to clearly elaborate the relationship between particle size and activity of Ni catalysts in the POM process. On the other hand, the porosity of the SiO₂ shell can be changed by adopting a specific surfactant such as PVP during SiO₂ encapsulation (Scheme 1C). Based on such a strategy, we prepared core–shell structured Ni@SiO₂ catalysts. The as-obtained core–shell catalysts were applied for POM at various temperatures. The effect of gas hourly space velocity (GHSV) on activity with time on stream was also studied. Techniques such as nitrogen sorption measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), hydrogen temperature-programmed reduction (H₂ TPR) and desorption (H₂ TPD), and temperature-programmed oxidation (TPO) were used to characterize the fresh and used catalysts. A structure–performance relationship has been established based on the results of characterization and evaluation.

Section snippets

Catalyst preparation

The NiO NPs were prepared by a chemical precipitation method. Typically, 2.9 g of Ni(NO₃)₂·6H₂O was dissolved in 40 mL of deionized water, and the solution was added dropwise into a solution that contained 100 mL of deionized water and 330 mg of polyethylene glycol (PEG) (average MW = 20,000, Fluka) and 1.0 g NaOH. The resulting solution was stirred for 1 h at room temperature (RT). The collected material was then washed several times with deionized water and ethanol, dried at 50 °C for 24 h, and

XRD

Shown in Fig. 1A are the XRD patterns of the NiO NPs obtained by calcining the nickel hydroxide precipitate at different temperatures. After calcination, the samples show only the fcc-NiO phase (JCPDS No. 78-0429), with typical reflections of the (1 1 1), (2 0 0), and (2 2 0) planes at 2θ = 37°, 43°, and 64°, respectively. With higher calcination temperatures, the crystallinity of NiO particles increases. After in situ reduction, the NiO entities were completely transformed into elemental Ni⁰, as

Concluding remarks

Silica-encapsulated Ni NPs with controllable size distribution (6–45 nm) were synthesized and applied for POM to syngas. The core–shell structured catalysts exhibit a narrow size distribution of Ni NPs and meso- or microporosity of SiO₂ shells. With increasing core size, the precursor NiO NPs are difficult to reduce. It is clear that the catalyst activity is dependent on the size of the Ni cores, the porosity of the SiO₂ shells, and the extent of core–shell interaction. The microcapsular

Acknowledgments

The financial support from NSFC (21173118), NSFJS (BK2011439), and the Ministry of Education for a special discipline grant of a doctoral supervisor (20110091110023) is greatly appreciated.

References (46)

P. Tijm et al.
Appl. Catal. A
(2001)
P. Prieto et al.
J. Catal.
(2010)
K. Takehira et al.
J. Catal.
(2004)
C. Berger-Karin et al.
J. Catal.
(2011)
T. Shishido et al.
Appl. Catal. A
(2002)
Z. Hou et al.
J. Catal.
(2007)
Y. Kobayashi et al.
Appl. Catal. A
(2011)
V. Choudhary et al.
J. Catal.
(1997)
J. Lu et al.
J. Catal.
(2005)
J. Lu et al.
J. Catal.
(2007)

K.T. Li et al.

Catal. Commun.

(2008)

S. Takenaka et al.

Appl. Catal. A

(2008)

S. Takenaka et al.

J. Catal.

(2007)

L. Yao et al.

Catal. Today

(2010)

K.S. Chou et al.

Micropor. Mesopor. Mater.

(2007)

Y. Li et al.

J. Catal.

(2006)

R.Y. Xie et al.

Catal. Commun.

(2011)

A. Boudjahem et al.

J. Catal.

(2004)

S. Takenaka et al.

J. Catal.

(2003)

J. Requies et al.

Appl. Catal. A

(2005)

T. Choudhary et al.

Catal. Lett.

(2001)

X.K. Li et al.

J. Catal.

(2005)

L. Chen et al.

Appl. Catal. A

(2005)

Cited by (151)

Ni-based core-shell structured catalysts for efficient conversion of CH<inf>4</inf> to H<inf>2</inf>: A review
2024, Carbon Capture Science and Technology
Methane (CH₄) serves as a primary constituent of natural gas and a potent greenhouse gas. Catalytic conversion of CH₄ into H₂ presents a promising avenue for mitigating CO₂ emissions from direct CH₄ combustion, while also facilitating the transition towards cleaner energy sources. Substantial efforts have been dedicated to enhancing the efficiency of methane conversion and the selectivity for hydrogen in catalyst design. Among these endeavors, core-shell structured Ni-based catalysts have emerged as highly advantageous catalysts for CH₄ conversion, with a multitude of benefits including high catalytic activity, coking resistance, and longtime stability. In this review, we provide an overview of Ni-based core-shell catalysts. Furthermore, the synthesis methods of core-shell Ni-based catalysts are explored. We then delve into the myriad advantages these core-shell structured catalysts confer upon methane conversion, specifically emphasizing their remarkable stability and resistance to sintering, and CO₂ activation capacity in dry reforming of methane. Moreover, this review comprehensively outlines the versatile applications of Ni-based core-shell structured catalysts in processes such as steam reforming of methane (SRM), dry reforming of methane (DRM), partial oxidation of methane (POM), and catalytic methane decomposition (CMD) for H₂ production. Additionally, we delve into the recent advancements in the field of computational chemistry, with a particular focus on density functional theory (DFT) and machine learning (ML). These techniques play pivotal roles in screening Ni-based catalysts and offer insights into the intricate reaction mechanisms underlying methane conversion. Finally, we summarize the key findings of this review and provide forward-looking perspectives. This comprehensive review serves as a valuable reference in the realm of hydrogen production from methane on core-shell structured catalysts, fostering a deeper understanding of this promising area.
MOFs-derived Ni@ZrO<inf>2</inf> catalyst for dry reforming of methane: Tunable metal-support interaction
2024, Molecular Catalysis
Metal-support interaction (MSI) plays an important role in heterogeneous catalysis, that could be well regulated by many strategies. Here, Ni@ZrO₂ catalysts were controllably synthesized by sacrificial Zr-MOFs template strategy, and were applied to the dry reforming of methane (DRM). Amongst, the Ni/ZrO₂-B catalyst shows the best performance toward CH₄ and CO₂ conversion, H₂/CO ratio, and stability, following by Ni/ZrO₂-A and Ni/ZrO₂C catalysts. The difference in catalytic performance should be ascribed to different synthetic strategy, resulting in different properties of Ni species and ZrO₂ matrix, and interface interaction. It revealed that small Ni particles were benefit for CH₄ activation, and strong MSI boosted CO₂ activation, that could remain the Ni dispersion and thermal stability. This study is beneficial to the catalyst design in DRM reaction.
Boosting oxygen vacancy formation and Ni dispersion by Ce<inf>0.9</inf>Y<inf>0.1</inf>O<inf>2</inf> nanorod: Efficient water and hydrocarbon activation
2023, Journal of Catalysis
Ni-based higher hydrocarbon steam reforming (SR) catalyst usually encounters low water dissociation rate and carbon deposition-induced inactivity and instability. Herein, we report a Ni–Ce_0.9Y_0.1O₂-nanorod (Ni–CYO–NR) with a MgSiO₃ postcoating that exhibits a TOF and conversion of 0.18·s⁻¹ and 95% at 700 ℃ and carbon deposition amount and deactivation rate constant 2 orders of magnitude lower than those of Ni–Ce_0.9Y_0.1O₂-nanoparticle (Ni–CYO–NP). Characterization results suggested that Ni–CYO–NR has a higher oxygen vacancy concentration and Ni dispersion than Ni–Ce_0.9Zr_0.1O₂–NR and Ni–CYO–NP because of its CYO(1 1 0) nature. DFT calculations revealed that CYO(1 1 0) has the lowest formation energy of oxygen vacancies and energy barrier for H₂O dissociation relative to CZO(1 1 0) and CYO(1 1 1). Additionally, MgSiO₃ postcoating inhibited the deep sintering of Ni, which in turn suppressed carbon formation. This demonstrates that the superior n-dodecane SR performance of MgSi–Ni–CYO–NR can be attributed to the enhanced and stabilized O–H, C–C, and C–H bond dissociation and carbon formation resistance and removal.
Syngas production through dry reforming of ethanol over Co@SiO<inf>2</inf> catalysts: Effect of SiO<inf>2</inf> shell thickness
2023, Molecular Catalysis
Morphology structure is regarded as one of the key factors in developing highly efficient catalysts for heterogeneous catalytic reactions. In this work, the influence of SiO₂ shell thickness on the performance of the Co@SiO₂ catalysts in ethanol dry reforming was studied in detail. Indeed, the different shell thickness resulted in various strengths of metal-support interactions for the Co@SiO₂ catalysts, thereby greatly affecting the activity/stability. Herein, Co@SiO₂–1.5 sample with ca.20 nm shell thickness exhibited better physicochemical properties. The O_ads/O_latt ratio of Co@SiO₂–1.5 (1.03) was higher than that of Co@SiO₂–1 (0.75) and Co@SiO₂–2.5 (0.91) as depicted in XPS spectra. O_ads sites were indexed with the activated O species originated from O defects. On the other hand, the highest performance was obtained over Co@SiO₂–1.5 sample with the optimal shell thickness as well as strong metal-support interaction. Particularly, 100% of ethanol conversion was achieved at 550 °C for Co@SiO₂–1.5 catalyst and the targeted H₂/CO ratio was closer to 1 compared to other samples. In addition, no obvious deactivation (ethanol conversion still be 100%) occurred after 40 h time on stream tests. This finding might depict a general strategy to design/develop the attractive catalysts with the core/shell structure for other heterogeneous reactions.
Engineering oxygen vacancy-rich CeO<inf>x</inf> overcoating onto Ni/Al<inf>2</inf>O<inf>3</inf> by atomic layer deposition for bi-reforming of methane
2023, Chemical Engineering Journal
Atomic layer deposition (ALD) was applied to develop CeO_x-overcoated Ni/Al₂O₃ catalyst for bi-reforming of methane (BRM), as the combination of dry reforming of methane (DRM) and steam reforming of methane (SRM). Non-stoichiometric CeO_x thin films were successfully deposited on Ni/Al₂O₃ particles by ALD, which constructed a beneficial Ni-CeO_x interface and modified the catalyst property. Ascribed to the unique ALD growth mode, a high amount of Ce(III) and oxygen vacancies existed in the ALD-deposited CeO_x overcoating. A reduction process before the BRM reaction contributed to the further reduction of Ce(IV) to Ce(III), resulting in more oxygen vacancies. The oxygen vacancies at the Ni-CeO_x interface enabled a high rate of CO₂ activation and enabled the balance between the activation of CO₂ and H₂O for BRM. Due to its oxygen vacancies as activation sites for CO₂ and H₂O, CeO_x ALD overcoating significantly improved the activity of Ni/Al₂O₃ catalyst and achieved a better control in the H₂/CO ratio with a suitable ratio of H₂O/CO₂/CH₄ feed. CeO_x overcoatings enhanced the reducibility of Ni(II) sites and assisted in preventing Ni from oxidation during the BRM reaction. Less carbon deposition was achieved by the Ni/Al₂O₃ catalyst with CeO_x overcoating as ascribed to its better reactant activation capacity.
High coke resistance Ni-SiO<inf>2</inf>@SiO<inf>2</inf> core-shell catalyst for biogas dry reforming: Effects of Ni loading and calcination temperature
2022, Fuel
In this study, Ni-SiO₂@SiO₂ core-shell catalysts were prepared using the ammonia evaporated synthesis method, followed by encapsulation by a silica layer. The effect of nickel content and calcination temperature on the catalytic activity were separately studied in the biogas dry reforming reaction. The results revealed that the catalytic activity improved and carbon deposition increased when nickel content increased from 2.5 to 15 wt.%. Ni-SiO₂@SiO₂ with 10 wt.% Ni loading exhibited stable and high methane and CO₂ conversions with negligible carbon formation during 12 h time on stream at 700 °C. Also, because of the high carbon resistance property of the prepared catalysts, the CH₄ conversion over Ni-SiO₂@SiO₂ catalyst remained constant at 52% for 12 h at a low reaction temperature of 600 °C. Compared with Ni-SiO₂@SiO₂ core-shell catalyst, Ni/SiO₂ supported catalyst fabricated by impregnation method exhibited lower activity and stability. High stability and carbon deposition resistance properties of the Ni-SiO₂@SiO₂ catalyst were ascribed to the strong Ni-SiO₂ interaction and coverage of the Ni particles by SiO₂, which make them resistant against migration and sintering. The results indicated that the catalytic activity gradually improved with the augment of calcination temperature from 600 to 800 °C. Ni-SiO₂@SiO₂ catalyst calcined at 800 °C with moderate nickel size and intermediate Ni-SiO₂ interaction demonstrated the best catalytic activity.

View all citing articles on Scopus

View full text