Imaging Gas-Phase Methyl Radicals over a Ag/SiO₂ Catalyst during the Partial Oxidation of Methanol

We report the operando imaging of the methyl radical mole fraction above a working catalyst. Using photofragmentation laser-induced fluorescence (PF-LIF), we imaged methyl radicals, an important intermediate species in hydrocarbon chemistry, in the near-surface gas phase over Ag/SiO2 catalysts during the partial oxidation of methanol. Methyl is detected within 2 mm of the surface at single-digit parts per million concentrations. The temperature dependence of the methyl mole fraction in the gas phase follows the methyl surface coverage predicted by computational models, while gas-phase formaldehyde shows a negative correlation with the predicted surface coverage. We propose that this positive or negative correlation with coverage is due to the different binding energies of the adsorbates. These results suggest that measurements of the near-surface gas phase can serve as a reporter of surface species and provide an understanding of the impact of gas-phase radicals in catalyzed chemical processes. For the studied catalytic system, we show that desorption of methyl into the gas phase should be included in future modeling efforts and speculate about the role of methyl radicals for the formation of C2 species. Methyl PF-LIF imaging could be used in a wide variety of gas conditions and chemistries, including oxidative coupling of methane in which methyl radicals play a central role.