During the methanol oxidation reaction carbon monoxide (CO) is formed, which strongly adsorbs onto the platinum catalyst, reducing the number of available reaction sites and thus the performance of the cell. The addition of other metals, such as ruthenium or gold, to the platinum catalyst tends to ameliorate this problem. In the case of platinum-ruthenium catalysts, the oxophilic nature of ruthenium is believed to promote the formation of hydroxyl radicals on its surface, which can then react with carbon monoxide adsorbed on the platinum atoms. The water in the fuel cell is oxidized to a hydroxy radical via the following reaction: H2O → OH• + H+ + e−. The hydroxy radical then oxidizes carbon monoxide to produce carbon dioxide, which is released from the surface as a gas: CO + OH• → CO2 + H+ + e−.