Study of Carbon Dioxide Transport in a Samaria Aerogel Catalyst by High Field Diffusion NMR

Pulsed field gradient (PFG) NMR employing a high magnetic field of 17.6 T was used to study self-diffusion of carbon dioxide in alumina stabilized samaria aerogel, a promising porous catalyst for gas-phase reactions. Such rare-earth aerogels exhibit high porosity and surface area with active sites directly integrated into the pore framework. In the reported diffusion NMR studies, application of a high magnetic field was essential for obtaining sufficiently high signal-to-noise ratios under conditions of relatively low CO2 densities in the primarily mesoporous catalyst particles. The diffusion studies were performed with the catalyst that was formed into the following two types of samples: macroscopic monoliths and beds of particles with sizes around 200 μm. The sorbate diffusivity inside the monolith was compared with the corresponding diffusivity in the bed under conditions of fast exchange of CO2 between the particles and the interparticle voids of the bed. The two-domain exchange model proposed by Karger for zeolites was used to describe the latter diffusivity. The reported results are expected to be useful for elucidating an influence of possible transport limitations under reaction conditions in aerogel catalysts.