Selective electrochemical oxidation of carbon by active oxygen for potential application as a sensor for diesel particulates

The formation of active oxygen by electrochemical oxidation of H2O vapor, coupled with carbon detection using those oxygen species, has been investigated by utilizing Sn0.9In0.1P2O7 as a proton conductor and Pt as an electrocatalyst. The H2O vapor was dissociated into protons and electrons at a Pt–carbon working electrode, which produced active oxygen on the carbon surface. This oxygen species showed high activity for carbon oxidation at temperatures of 50 °C or higher, where CO2 formation increased with temperature and reached the theoretical value calculated from the four-electron reaction (C + H2O → CO2 + 4H+ + 4e−) at 200 °C. When the formation rate of active oxygen was greater than the oxidation rate of carbon at the working electrode, the electrode potential increased rapidly due to a significant increase in polarization resistance. The timing of the potential jump was strongly dependent on both the carbon content in the working electrode and the current to the working electrode, which provides information on the quantity of carbon. In addition, little interference was encountered in sensing for carbon at 1000 ppm CO and 1000 ppm C3H8 in the sample gas. The high selectivity toward carbon was based on different mechanisms for CO and C3H8; almost all of the CO was catalytically oxidized to CO2 before reacting with active oxygen, while C3H8 experienced little reaction with active oxygen. These results demonstrate that the present sensor sensitively and selectively detects carbon while self-regenerating.