Not so inert gases: Substantial weight gain by oxidation of a porous SiCO gel under N2, Ar, and He atmospheres

We investigate the pyrolysis of a polymer-derived porous SiCO gel through thermogravimetric analysis. Despite using “ultra-pure” (99.999%) Ar, N 2 , or He, we observe a substantial apparent mass gain of 4 percentage points at temperatures above 700 °C and below 900 °C. Chemical analysis and FT-IR characterization of the pyrolyzed material attribute the effect to oxidation. By implementing additional gas purification steps, we reduce or even eliminate the mass gain, identifying the “inert” gas used during pyrolysis as the source of oxidation. As the porous SiCO gel decomposes, it becomes hypersensitive and reacts almost quantitatively with trace impurities in the gas stream. At 800 °C, the oxidation by trace impurities blocks a fraction of mesopores and reduces the surface area of the sample. The material presented here can serve as a probe to test the inert environment used in thermal experiments. Moreover, similar porous silicon oxycarbide precursors may have undergone oxidation that went unnoticed.