Residual hydrogen in supported platinum catalysts and its influence on their catalytic properties

When Al2O3-, SiO2-, or TiO2-supported Pt catalysts were reduced above 400°C, there was a sharp decrease in their hydrogenolysis activity for n-pentane at 400°C. The chemisorption of H2 on the catalyst at 20°C also decreased as if a part of the surface Pt atoms was no longer accessible to chemisorption. A few air-H2 cycles at 20°C restored part of the hydrogenolysis activity and H2 chemisorption capacity lost on reduction of the catalyst at higher temperatures. An air oxidation at 500°C followed by reduction at 400°C fully restored the original properties of the catalyst. The uptake of hydrogen by oxidized catalysts above 400°C was more than would correspond to the topmost layer of the Pt surface only. In the case of Pt-TiO2, a reduction of the carrier was shown to occur at about 500°C by means of temperature-programmed desorption and reduction studies. The attenuation of the catalytic properties of the three types of catalysts studied seems to be caused mainly by a stronger chemisorption of H2 on Pt since such effects are observed on Pt-b1ack also. Self-inhibition by strongly chemisorbed H2 seems to be a widespread phenomenon in catalysis.