(544ay) Exploiting Pore Diffusion in Core@Shell Nanocatalysts

Authors: 
Yang, Y., University of Pittsburgh
Veser, G., University of Pittsburgh
The engineering of materials on the nanoscale enables precise tailoring of materials’ functionality. Core-shell materials are a widely studied class of engineered nanomaterials with application in various technologies. In catalysis, core-shell nanostructures have drawn much attention due to their excellent thermal stability. From a reaction engineering perspective, these nanocatalysts can also be considered “nano-reactors” with porous membrane walls for preferential diffusion of molecules, and hence the ability for tuning of selectivity by tailoring the porosity of the shell material.

In the present work, we designed nickel-silica based core-shell nanostructured catalysts (Ni@SiO2) with SiO2 pore diameters of ~ 0.8-1.2 nm. Fine control of SiO2 shell thickness with near nanometer precision can be achieved by adjusting synthesis parameters, including reaction time and SiO2 precursor concentration, which allows us to control the degree of preferential diffusion and evaluate this effect in a rational way.

The impact of pore diffusion in Ni@SiO2 core-shell nanostructures is studied using methane oxidation as model reaction. We find that at oxygen-rich conditions (CH4:O2 feed ratio=1:5), increasing SiO2 shell thickness can slow down NiO formation and hence slow down catalyst deactivation. This can be explained by the delayed diffusion of O2 due to increased diffusion length and hence a less oxygen-rich gas mixture in the cores, which furthermore demonstrates the presence of strong pore diffusion limitation in Ni@SiO2. Moreover, we studied the preferential oxidation of H2 and CH4 over pre-oxidized Ni@SiO2 where gaseous O2 is replaced by lattice oxygen, allowing us to identify the preferential diffusion between H2 and CH4. We observe that H2 conversion indeed precedes CH4 conversion by 0.4 minutes, confirming the existence of significant preferential diffusion of H2 molecules. This “sieving effect” can be traced back to configurational diffusion in the porous SiO2 matrix, confirming the potential use of core-shell nanomaterials as tunable catalysts. We are currently extending the application of these core-shell nanocatalysts in oxidative dehydrogenation of ethane to ethylene.

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