(544cj) Diffusion of Light Gases in Nanoporous Gold By Pulsed Field Gradient NMR

Nanoporous gold with pores in the mesopore range has been widely explored for various catalytic applications. Simple fabrication of nanoporous gold along with high specific surface area, tunable framework and catalytic activity has led to a significant recent interest in this promising catalyst. As one specific example, nanoporous gold has shown to be a promising candidate for the catalytic oxidation of carbon monoxide to carbon dioxide. Fundamental understanding of molecular transport in nanoporous gold is of crucial importance for understanding and further optimizing its catalytic properties. The transport properties of mesoporous catalysts can be characterized by the tortuosity factor, which can be defined as the ratio of the diffusivity in a well-defined reference system (bulk diffusivity) and the diffusivity inside a porous catalyst.

Carbon-13 pulsed field gradient (PFG) NMR was used to study self-diffusion of carbon monoxide, methane and carbon dioxide in a bed of nanoporous gold particles at 296 K. For methane, selected measurements were also performed using ¹H PFG NMR to confirm the absence of any measurement artefacts under our experimental conditions. Diffusion measurements were performed for a broad range of displacements that were smaller and larger than the smallest dimension of nanoporous gold particles. For displacements smaller than the particle size, the following two ensembles are resolved for each studied sorbate: (1) molecules that diffuse inside the particles, and (2) molecules that diffuse in the gas phase of the sample outside the particles. The observed two ensembles have two different diffusivities where the ratio between the two diffusivities can be defined as the tortuosity factor. Within an experimental uncertainty, this tortuosity factor was found to be the same for each studied sorbate. This result indicates that under our experimental conditions there were no strong gas-pore wall interactions that can prevent measurements of the true tortuosity factor. This work represents the first study of microscopic gas diffusion in nanoporous gold by any experimental technique. Quantification of gas diffusion in this work is expected to be useful for optimizing catalysis involving gas molecules in nanoporous gold.