Monodispersed Ni and Ni-Co Nanoparticles Capsulated with Silica for Dry Reforming of Methane

Abstract: To figure out the intrinsic effect of Co to the enhanced activity and stability in Ni catalysts for the dry reforming of methane (DRM), the nanoreactor of Ni-Co@SiO₂ catalysts were synthesized, which are composed of Ni-Co nanoparticles encapsulated in the SiO₂ shell. The steric hindrance to the sintering was induced from the silica shell where the nanoparticles could retain their pristine size at the high temperature dry reforming of methane without serious deactivation as supported Ni catalysts showed. The optimum molar ratio of Ni/Co was determined in terms of DRM activity and stability. Also mechanisms of synergetic effect of Ni and Co are proposed by the various characterizations such as TPR, in-situ IR, EXAFS and others.

Keywords: Nanoreactor, Ni-Co nanoparticles, SiO₂ coating, Core-shell catalysts, Dry reforming of methane (DRM), Oxophilic cobalt

1. Introduction

Dry reforming of methane (DRM) is nowadays spotlighted in environmentally and industrially since it could convert two main greenhouse gases, methane and carbon dioxide which are responsible for global warming and could produce syngas ratio close to 1, which is favorable for the further conversion to the synfuels. In addition the shale revolution, which increased the production of natural gases tremendously, made the process more feasible which converting the C1 gases to liquid fuels and chemicals such as methanol and synthetic gasoline [1].

Two main reactions in DRM process as follow: Methane decomposition, Boudouard reaction.

(R1) Methane decomposition : CH₄ -> C(s) + 2H₂
(R2) Boudouard Reaction : 2CO = C(s)+CO₂

During the reaction, it is pointed that the CH₄ is dissociated to be hydrogen and solid carbon on the active metal, which can be gasified by the CO₂ molecule to 2 molecules of CO. Since this solid carbon can deposit on the surface of active metals, it can harm the stable production of the syngas by catalyst deactivation and reactor blocking. The DRM process is usually done at high temperature of 700 – 900 ^oC to accelerate the gasification of the coke so that the long life time of the catalysts is granted.

Generally Ni catalysts are favored which is far cheaper than the precious metal based ones with comparable activity on the reforming processes. Major drawback of the Ni catalysts is the process stability. Since Ni nanoparticles are easily sintered to make bigger agglomerates, where the coke can easily deposit onto, the Ni catalysts are easily deactivate in the high temperature DRM condition [2]. Efforts are being made by many researchers continuously to increase the stability of Ni catalysts. One of the promoters can be used for the Ni catalysts is Co, which is known to promote the activity and stability of the catalysts with proper amount of introduction to the catalysts. Co itself is not active for the reforming process as Ni is, however the oxophilic nature of Co which will easily utilize the surface oxygen to facilitate the gasification of the coke during the reaction leading to the promotion of activity and stability of the catalysts [3]. Some researchers noted that the synergetic effects of Ni-Co are induced from the formation of Ni-Co alloy where the Ni can be electron-rich from the electron donation of Co upon alloy formation [4]. However the formation of Ni-Co alloy was hardly observed directly instead of only ex-situ characterizations which cannot simulate the characteristics of catalysts situated in the high temperature DRM reaction.

Via the rational design of nanoreactor, which preserving the state of metal nanoparticles composed of Ni and Co during the DRM reaction, it is possible to observe the synergetic effect of Ni and Co which can be hardly confirmed in the conventional supported catalysts due to its heterogeneity nature. From the encapsulation of Ni-Co alloyed nanoparticles in the silica shell, it is possible to lessen the phenomena from sintering problems, metal-support interactions and coke deposition whereas the pristine synergetic effect of Ni-Co can be directly observed.

In this research, nano-reactor of Ni-Co@SiO₂ multi core-shell catalysts were prepared and evaluated in terms of DRM reaction to confirm the synergetic effect of Ni-Co nano-alloys and to figure out the origin of the effects by means of various characterizations.

2. Experimental

Ni-Co@SiO₂ multi core-shell catalysts are prepared by two main steps of nanoparticle synthesis and encapsulation of the nanoparticles with SiO₂.

Ni and Ni-Co nanoparticles are prepared from the thermal decomposition of precursors by hot injection method. Firstly, regarding to the targeted Ni/Co ratio, the determined amounts of nickel acetylacetonate and cobalt acetylacetoate of 2 mmol in total were dissolved in the solution of 3 mL oleylamine and determined amounts of oleic acid and trioctylphosphine. The solution was degassed with bubbling of UHP N₂ at 110 ^oC for more than 0.5 h. After the 25 mL of oleylamine solvent was heated to the injection temperature ranged among 240 - 250 ^oC as targeted Ni/Co ratio with bubbling of N₂ in the oil bath, the degassed precursor solution is rapidly injected to the solvent to nucleate the Ni and Co at the same time. After the temperature of the mixture reached to the injection temperature, the temperature was maintained for appropriate time to grow the nanoparticles sized about 6 nm in diameter. The mixture was cooled in the room temperature until the temperature reached to 100 ^oC where the 40 mL of ethanol was added to the mixture to separate the nanoparticles from the solvent. The nanoparticles are collected and washed via the centrifugation with ethanol as washing agent. The washed nanoparticles are dispersed in the cyclohexane for further coating of SiO₂.

The encapsulation of nanoparticles was done via the reverse micelle method. Nanoparticles-dispersed cyclohexane and the solution composed of IGEPAL CO-630 and 35 mL of cyclohexane were sonicated for more than 20 min. Then, the solutions are mixed together and stirred vigorously overnight to reach the equilibrium of ligand exchange from OAc and TOP to CO-630 on the surface of nanoparticles. 2mL of tetraethoxy silane (TEOS) was injected slowly as a silica source right after the NH₄OH was added drop by drop to hydrolyze the TEOS. After the reaction mixture was stirred for 72 h, the prepared nanoparticles were collected by means of centrifugation following the addition of 20 mL methanol to separate the particles from cyclohexane. The collected particles are further washed with hexane and methanol subsequently for few times followed by drying at 80 ^oC convection oven overnight. Dried ones were crushed and calcined at the 450 ^oC and 800 ^oC subsequently for 0.5h and 2h respectively at a ramping rate of 1^oC/min. The catalytic activity of Ni-Co@SiO₂ catalysts were reduced in-situ at the 800 ^oC for 1 h and tested in a 3/8 inch fixed-bed reactor at the following reaction conditions; T = 800 ^oC, P = 0.1 MPa, gas hourly space velocity (GHSV) = 36,000 L/kg_cat∙h and the feed molar ratio of CH₄/CO₂/N₂=1/1/2. The effluent products were analyzed using online gas chromatography equipped with TCD.

The catalysts are characterized with various tools such as XRD, TPR, XPS, TEM, ICP-AES, In-situ IR, XAFS and so on to clarify the mechanism of the synergetic effect of Ni and Co for the DRM reaction.

3. Results and discussion

The Ni-Co nanoparticles with different molar composition of Ni and Co were successfully synthesized to have the monodispersed sizes of 5-6 nm of diameters. Since the injection temperature was set to decompose Ni and Co at the simultaneously but to attain the small sized nanoparticles less than 10 nm, the ratio of Ni/Co was a lot bigger than as targeted. Still it was possible to increase the contents of Co in the particles gradually with increasing the amount of the cobalt precursor put. After the silica coating process, the nanoparticles retained their sizes after the high temperature reduction at the temperature of 800 ^oC with successfully structured as multi core-shell Ni-Co@SiO₂ nanocatalysts. However the catalytic activity and stability towards DRM is changing with the molar ratio of Ni/Co. Especially the specific rates of conversion with regard to the amount of metals showed the volcano shaped trends where the optimum existed. The research is still under investigation. Up to now the ratio of Ni/Co is supposed to be related to the conversion of CO₂, accelerating the decomposition of CH₄ on the Ni since it promotes the gasification of carbons to CO. With the combination of the results of In-situ IR and other characterization tools, it would be possible to disclose the main roles of Co used with Ni and the phenomena involved in the DRM process.

References

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