Silica Encapsulated Core-shell Nanostructured Metallic Catalyst Synthesis And Application In CO_x-free H₂ Production Via NH₃ Decomposition

The on-site generation of hydrogen is essential for proton-exchange membrane fuel cells (PEMFCs). The hydrogen directly obtained from carbonaceous compounds such as methanol and methane has its limitations because the COx (x= 1,2) side products can damage the cell performance even at extremely low concentrations. COx-free hydrogen obtained from catalytic ammonia decomposition has been an alternative route because ammonia can be easily liquefied and thus its storage and delivery can be readily handled.The application of nanoparticles (NPs) in heterogeneous catalysis is highly desirable due to the intrinsic "surface effects". Unfortunately, NPs are unstable and aggregate easily especially at elevated temperatures. Core-shell structured materials have attracted great attentions in recent years because of the unique structural feature and physicochemical properties. Particularly SiO2 encapsulated nanostructures have been studied and applied in many fields such as biology, optics, electronics, magnetism and sensing. Enwrapping a nano-material in a stable but porous shell can enhance the stability of the core material and in addition, may cause a change in electron charge, reactivity and functionality of the enwrapped material.In the present study, microcapsular-like nanostructures are designed as the novel catalysts for the production of COx-free H2 through NH3 decomposition. It was found that the catalytic performance can be further tuned by changing the parameters in the synthesis procedure. Together with the characterization of the catalysts and their catalytic performance, the major conclusions can be provided as follows:1. The core-shell structures can be synthesized by a two-step method. The core particles can be prepared via a precipitation or reduction route, the micro/meso-porous SiO2 shell can be formed through a sol-gel approach. After reduction in-situ in hydrogen, the core-shell catalysts can be obtained.2. The catalytic performance is also highly related to the chemical nature of core component. It was found that as for ammonia decomposition Ru is the most active among the noble metals while Fe and Co based catalysts are comparable in activity.3. The obtained core-shell catalysts show superior activity and stability to the naked cores or the supported counterparts, which may arise from their unique structure features. In order to further understand the influence of the space formed between the core and the inner side of the SiO2 shell, we designed two Ru based core-shell catalysts, and found that the microcapsular Ru@SiO2 is significantly more active than the fully filled core-shell Ru@SiO2. The microcapsular core-shell structured NPs can provide a unique environment around the cores and function as microcapsular-like reactors in which the reactant molecules can be enriched. The consequence is enhanced adsorption and reaction on the core, and higher catalytic activity.4. By employing C18 TMS, the porosity of the SiO2 shells can be tuned. It was found that the mesoporous SiO2 coated catalyst presents much better performance than the microporous SiO2 coated one, indicating that the texture of the shells plays an important role in molecule diffusion, and thus influence the activity. 5. In the present study, we found that introduction of certain kinds of alkali, alkaline-earth, and rare earth elements can enhance the catalytic activity. However, the method of promoter introduction plays an important role in determineing the catalytic behavior. Take the Cs modified iron based catalysts as an example. For the microporous SiO2 encapsulated catalyst, the activity can be enhanced when Cs was introduced through a "pre-deposition" approach, while the activity declines by using a "post-impregnation" method. For the mesoporous SiO2 coated one, the "post-impregnation" method can also enhance the catalytic performance. The results further demonstrate that the porosity of shell is crucial for the core-shell type catalyst.