The Catalytic Oxidation of Carbon Monoxide on Titanium Dioxide; Anatase and Rutile

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Abstract
For the purpose of investigating the correlation between the crystal structure of titanium dioxide and its catalytic activity, the catalytic oxidation rate of carbon monoxide and the desorption rate of oxygen from the sample surface have been measured on anatase, rutile, and an anatase-rutile mixture. The oxidation of carbon monoxide was investigated by a static method at temperatures of 250–550°C and under gas pressures of 10−2–150 mmHg. The desorption of oxygen was carried out at 600–750°C. The reaction rate was proportional to PCO and did not depend on either PCO2 or PCO2 The activation energy was 16.0 kcal/mol for anatase and 6.7 kcal/mol for rutile. The catalytic activity per unit of the surface area of rutile was 3—6 times as much as that of anatase. The reaction rate almost coincided with the rate of the reduction of titanium dioxide by carbon monoxide. The electric resistance of the sample was little affected by the PO2. The following reaction mechanism is proposed to explain the results. The rate-determining step is the reaction between the surface oxygen of the sample and the carbon monoxide physisorbed or that which has just collided with the surface coming from the gas phase; the oxygen vacancy thus formed on the surface is quickly filled with oxygen of the gas phase. The difference in the activation energy of the catalytic action between the two structures is based upon the difference in the activation energy of the oxygen desorption from the surface structure; the activation energies are 57.8 kcal/mol for anatase and 38.4 kcal/mol for rutile.