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