Nickel catalyst with atomically-thin meshed cobalt coating for improved durability in dry reforming of methane
Graphical abstract
Introduction
Recent advances in natural gas recovery and applications have drawn much attention to the dry reforming of methane (DRM) with carbon dioxide (CO2) [1], [2], [3]. The DRM reaction provides an opportunity to convert CO2 with another abundant greenhouse gas CH4, to produce syngas with a H2/CO ratio close to 1, which is potentially favorable for other valuable chemicals synthesis [4], [5], [6]. While nickel (Ni) nanoparticles are attractive catalysts for DRM reactions in terms of activity and cost, its practical application is plagued by the quick deactivation in the catalytic environment [7], [8]. Reforming reaction is highly endothermic and typically run at high temperature above 800 °C, during which Ni catalysts display a remarkable decrease in activity. Sintering and coking formation are two main causes of performance degradation [9]. Sintering decreases the metal surface area, while coking causes thick carbon network formation that alters reactants flows in the reactor and blocks active sites of catalysts [10], [11].
Previous works have shown that composite Ni catalysts could improve the stability for DRM reaction [12], [13]. Several strategies have been developed that focus on the effect of supports, surface passivation of Ni’s coking sites, and Ni-based alloy catalysts [14], [15]. Surface decorators, such as Sn and S, limit coking by decreasing the carbide formation and prevent carbon nucleation on Ni catalyst. It is found that coke deposition can be suppressed when the Ni catalyst is Sn over doped (Sn/Ni > 0:02). However, the reforming activities of the best Sn0.02Ni catalyst decrease more than 20% [16], [17]. Previous works also show that composite Co/Ni alloy catalyst decreases the ensemble size of continuous Ni regions and limits the amount of carbon whisker growth due to the low solubility of carbon and the low adsorption strength of carbon on Co [18], where the Co/Ni composite surface structures are critical to the final catalytic performance and coking resistance properties. However, the intermediate species generated during the DRM reaction will gradually attract Co to segregate onto the alloy surface [19], thus overloaded Co may cause lacking of active sites on Ni-Co alloy surface thus decreasing the DRM activity, as Co itself is inactive towards CH4 dissociation [20], [21], [22], [23]. Excessive Co may also cause the Ni suffering from oxidation of active metallic sites [24], [25]. Meanwhile, in alloy Co-Ni catalysts, the near surface is randomly composed of Co and Ni component, continuous carbon whiskers formation cannot be avoided on continuous Ni atoms packing region as Co packing arrangement is random. A Ni-based structure with ultra-low Co loading and an atomic level control over morphology is highly desired to develop highly active and coking resistant DRM catalysts [26], [27].
In this work, meshed-like Co coating catalytic structure is designed and fabricated to decorate Ni nanoparticles via atomic layer deposition (ALD). In the ALD process, two gaseous precursors are pulsed alternately and reacts with previous chemisorbed precursor on the substrate. With such self-limiting and layer-by-layer growth properties, it provides the advantages to form ultra-thin film with atomic-scale thickness control. In this work, the catalysts are achieved via a post reduction of as deposited atomically thin discontinuous CoOx layers on Ni via ALD, where oxygen release from CoOx with different crystalline structures produces a meshed Co coating. The ALD prepared catalysts show enhanced DRM reactivity and coking resistance simultaneously. The meshed coating structures partition the Ni catalysts' surface to prevent continuous generation of carbon nanotubes network. No carbon whiskers is observed on meshed catalysts after extended reaction duration at 650 °C (most severe carbon deposition temperature rage). The coking amount can be reduced over 2 to 6 times compared with a series of state-of-the-art research reports (8–23%) on Ni-based catalysts in the similar reaction temperature. An optimized catalytic performance (Ni:Co = 12:1, 0.50 Co wt%) can be achieved by carefully tuning the continuity and thickness of the Co coating layer. The Co coating layer provides physical confinement and also improves the thermal stability of Ni nanoparticles from sintering and agglomeration up to 850 °C.
Section snippets
Catalysts preparation
Ni supported on Al2O3 catalysts were prepared by wet impregnation method. Nickel nitrate was dissolved in deionized water and impregnated into γ-Al2O3. The Ni loading was fixed to 6 wt% for the reduced samples. The catalysts were dried at room temperature for 12 h and at 120 °C overnight. Then the samples were ground and calcinated at 550 °C in the air for 6 h, and naturally cooled to room temperature.
The prepared Ni/Al2O3 were reduced at 650 °C for 3 h under 10% H2 in N2 atmosphere and then
Structures of Co/Ni composite catalysts
In order to retain the catalytic activity during DRM reaction, one of the key points in the catalyst preparation is to obtain a discontinuous coating structure that facilitates the accessibility of reaction gases to Ni’s surface. The Co layers are fabricated with different coating thickness (3, 6, 9 ALD deposition cycles). From the calibration of ICP measurements, the coating cycles are converted to Co loading of 0.25, 0.50, 0.75 wt%, respectively. The coating thickness (or the mass gain) is
Conclusions
In summary, mesh-like Co coating catalytic structure is designed and synthesized to decorate Ni NPs via atomic layer deposition with improved durability for dry reforming of methane (DRM). The ALD fabricated catalysts are highly active and coking resistant. The activity and coking resistance are influenced by the coating layer configuration, and optimized catalytic performance is achieved with properly tuning the continuity of the Co coating layer. The meshed coating structures passivate and
Acknowledgements
This work is supported by the National Natural Science Foundation of China (51702106, 51871103, 51575217, 51835005 and 51572097). The authors acknowledge the Analytical and Testing Center, the Flexible Electronics Research Center of Huazhong University of Science and Technology.
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