Ethyl radical ejection during photodecomposition of butanone on TiO2(1 1 0)

The photodecomposition of acetone and butanone were examined on the (1 1 0) surface of rutile TiO2 using temperature programmed desorption (TPD) and photon stimulated desorption (PSD). In both cases, photodecomposition was preceded by a required thermal reaction between the adsorbed ketone and coadsorbed oxygen resulting in an adsorbed diolate species. The diolate photodecomposed by ejection of an organic radical from the surface leaving behind a carboxylate species. In the acetone case, only methyl radical PSD was detected and acetate was left on the surface. In the butanone case there was a possibility of either methyl or ethyl radical ejection, with propionate or acetate left behind, respectively. However, only ethyl radical PSD was detected and the species left on the surface (acetate) was the same as in the acetone case. The preference for ethyl radical ejection is linked to the greater stability of the C–CH3 bond in butanone over that of the C–C2H5 bond. Unlike in the acetone case, where the ejected methyl radicals did not participate in thermal chemistry on the TiO2(1 1 0) surface after photoactivation of the acetone diolate, ethyl radicals photodesorbing at 100 K from butanone diolate showed preference for dehydrogenation to ethene on the surface through the influence of coadsorbed oxygen. These results reemphasize the mechanistic importance of organic radical production during photooxidation reactions on TiO2 surface.