Study of reactor-NO2-gas diffusion in a porous glass chip by near-infrared Raman spectroscopy

We have used near-infrared (NIR) Raman spectroscopy to study a coloration reaction that occurs in nano-sized pores in a porous glass chip. This coloration reaction is a modified Saltzman reaction that produces an azo dye by selective reaction with nitrogen dioxide gas (NO2). Neither the details of the reaction mechanisms in the nano-sized pores nor the interaction between the products and the pore surface have previously been investigated. We analyzed the mechanism by measuring the Raman intensity of the azo dye produced in the pores. We found that, in the early stages of NO2 exposure, the NO2 gas reacts at the surface of the porous glass chip, while in the later stages more NO2 gas diffuses into deeper areas without reacting with the reagent. We estimated the limit of the NO2 gas diffusion distance to be about 500 μm. We also analyzed the interaction between the azo dye and the surface of the pores by comparing the Raman spectrum in the porous glass chip with that in a bulk d4-methanol solution. The azo dye is mainly adsorbed as a result of hydrogen bonding between the π-electrons of the phenyl group and the silanol of the glass surface. We also determined that the azo dye has a trans-conformation molecular structure by calculating the total steric energy. The interaction between the azo group and the silanol is weaker in the adsorbed state than in bulk, due to the steric effect of the bulky substituents around the azo group. Our proposed geometry for the azo dye adsorbed on the surface of the pores has a flatter orientation.