(560im) The Development and Application of Novel Yolk@Shell Materials to CO2 Recycling

The yolk@shell materials detailed here were designed to confront two key areas of catalytic deactivation: coking and sintering. Coking is known to occur in a number of ways: adsorption to the surface through a “monolayer-multilayer” mechanism[1]–[3], total encapsulation of the material that completely deactivates the catalyst [3] or fill micro- and mesopores to prevent or at least limit access of the reactants to the active sites [3]. Sintering, too, occurs in several ways: reduction in active surface area due to the growth of the catalytic phase, reduction in the amount of support area due to the collapse of the support phase in addition to the collapse of the porous structure [4].

To prevent these problems, the mesoporous-SiO₂ shell, within which our active core is held, provides a physical shield to mitigate sintering and acts as a barrier to prevent the blocking of the active sites through coke formation.

Our catalysts were tested in the dry reforming of methane (DRM) and Reverse Water Gas Shift (RWGS) reactions due to renewed interest in the “green” chemistry approach to produce added value products through chemical recycling CO₂. These reactions were also selected for their high temperature requirement that cause catalysts to be prone to sintering and coking mechanisms [5]–[8].

Herein, we report the successful synthesis of two Ni-based Yolk@shell catalysts: Ni/ZnO@m-SiO₂ for the DRM reaction and NiCo@SiO₂ towards the RWGS reaction. The NiZnO@m-SiO₂ catalyst was found to display considerable long-term activity over 143 hours, far surpassing standard supported metal nanoparticle materials tested under the same conditions. The post reaction characterization of the catalysts detailed no discernable sintering of the Ni nanoparticles inside the shell and SEM confirmed the successful protection of the core from carbon deposition.

Several NiCo@SiO₂ materials were produced with varied Ni content; 2, 5 and 10 wt%. These materials were applied to the methanation/RWGS reaction. Initial reactive testing has found that all these materials displayed considerable low-temperature activity towards the RWGS reaction. Interestingly, the results suggest that the Ni loading has a greater effect on the selectivity of the material as all the catalysts displayed similar levels of CO_2­ conversion. Further studies of these materials are currently underway to determine the effect of property variation. Subsequent stability testing of the 2 wt% Ni catalyst, revealed that these materials are capable of significant long-term activity at 600^oC, detailing no sign of deactivation over an 89-hour study.

In conclusion, the novel morphology of these materials affords long term protection from the reactive environments faced by all catalysts. The benefits of the morphology are clearly presented when considering the comparison of our materials to two reference materials (Ni/Al₂O₃ and Ni/SiO₂-Al₂O₃) with comparable Ni content in the DRM, clearly displaying long term isothermal performance and sintering resistance, while still providing substantial levels of CO₂ conversion. Further study regarding the NiCo@SiO₂ materials is expected to detail further enhancement and selectivity over existing catalysts at substantially lower temperatures.

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