Imaging Gas-Phase\nMethyl Radicals over a Ag/SiO<sub>2</sub> Catalyst during the Partial\nOxidation of Methanol

We report the operando imaging of the methyl radical\nmole fraction\nabove a working catalyst. Using photofragmentation laser-induced fluorescence\n(PF-LIF), we imaged methyl radicals, an important intermediate species\nin hydrocarbon chemistry, in the near-surface gas phase over Ag/SiO<sub>2</sub> catalysts during the partial oxidation of methanol. Methyl\nis detected within 2 mm of the surface at single-digit parts per million\nconcentrations. The temperature dependence of the methyl mole fraction\nin the gas phase follows the methyl surface coverage predicted by\ncomputational models, while gas-phase formaldehyde shows a negative\ncorrelation with the predicted surface coverage. We propose that this\npositive or negative correlation with coverage is due to the different\nbinding energies of the adsorbates. These results suggest that measurements\nof the near-surface gas phase can serve as a reporter of surface species\nand provide an understanding of the impact of gas-phase radicals in\ncatalyzed chemical processes. For the studied catalytic system, we\nshow that desorption of methyl into the gas phase should be included\nin future modeling efforts and speculate about the role of methyl\nradicals for the formation of C<sub>2</sub> species. Methyl PF-LIF\nimaging could be used in a wide variety of gas conditions and chemistries,\nincluding oxidative coupling of methane in which methyl radicals play\na central role.