Structure-sensitive interactions of Pt/SiO2 with small molecules. Activation by oxygen and deactivation by carbon monoxide for methylcyclopropane hydrogenolysis

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Abstract
The hydrogenolysis of methycyclopropane (MCP) has been used as a probe reaction to investigate the structure sensitivity of the interactions of a series of well-characterized platinum on silica catalysts with oxygen and carbon monoxide. A differential flow reactor combined with a pulse doser has been used to obtain turnover frequencies for the product n- and iso-butane at 273 K and feed composition ca. 0.05 mol fraction MCP. Turnover frequencies for the sereis of Pt/SiO2, ranging in percentage of exposed metal from 7.1 to 81, are substantially increased as a result of a standardized oxygen pulse pre-treatment as a final step before MCP reaction, and the effect is structure sensitive. Activity is most enhanced on those catalysts of highest percentage of exposed metal and is also a function of oxygen adsorption temperature, being most pronounced above 473 K. However, selectivity, defined as (i-C4/n-C4), is decreased by oxygen pretreatment. The same series of Pt/SiO2, though, is poisoned by carbon monoxide for the MCP hydrogenolysis under the same conditions. Again the effect is structure sensitive, with intermediate range percentage exposures preferentially retaining activity for carbon monoxide coverages > 40%. Selectivity is not affected. Carbon monoxide pulse chemisorption experiments indicated a 1:1 adsorption stoichiometry for CO:Pt in this series at 296 K.

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