examples is the oxidation of ethylbenzene, yielding acetophenone that is an important raw material for producing medicines, fragrances, cellulose ethers, resins, flavoring agents, and many others. [6][7][8][9] However, the CH bond of ethylbenzene is so strong that its activation and cleavage usually require harsh conditions such as high temperatures. [10][11][12][13][14][15] In general, tert-butyl hydroperoxide or hydrogen peroxide is utilized to create hydroxyl radicals (•OH) with the help of a catalyst. •OH is highly reactive and can easily oxidize ethylbenzene. Nonetheless, the reaction also creates phenethyl alcohol and other byproducts, resulting in reduced selectivity with respect to acetophenone. [16][17][18][19] Given the great utility of this organic compound, it is necessary to develop strategies for improving the reaction selectivity under mild conditions. As a type of sustainable technology, photocatalysis that is able to use solar energy to generate reactive species has already shown tremendous potentials in organic synthesis.Polymeric carbon nitride (CN) as a metal-free photocatalyst has captured widespread attention. [20][21][22][23][24] Under light illumination, free electrons [25][26][27][28] and holes [29][30][31][32] are generated in CN, Selective oxidation of CH bonds is one of the most important reactions in organic synthesis. However, activation of the α-CH bond of ethylbenzene by use of photocatalysis-generated superoxide anions (O 2 •− ) remains a challenge. Herein, the formation of individual Fe atoms on polymeric carbon nitride (CN), that activates O 2 to create O 2 •− for facilitating the reaction of ethylbenzene to form acetophenone, is demonstrated. By utilizing density functional theory and materials characterization techniques, it is shown that individual Fe atoms are coordinated to four N atoms of CN and the resultant low-spin Fe-N 4 system (t 2g 6 e g 0 ) is not only a great adsorption site for oxygen molecules, but also allows for fast transfer of electrons generated in the CN framework to adsorbed O 2 , producing O 2 •− . The oxidation reaction of ethylbenzene triggered by O 2•− ions turns out to have a high conversion rate of 99% as well as an acetophenone selectivity of 99%, which can be ascribed to a novel reaction pathway that is different from the conventional route involving hydroxyl radicals and the production of phenethyl alcohol. Furthermore, it possesses great potential for other CH activation reactions besides ethylbenzene oxidation.

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applied in various fields including mining, aviation, medical diagnostics, food safety, automobile industry, indoor air quality monitoring, and environmental pollution control, etc. [1][2][3][4][5][6][7] Among them, due to the close relationship with human health, the applications of gas sensors in indoor air quality monitoring, breath analysis, and food safety have received wide attentions. [8][9][10] Conventionally, several