Model Chromia Surface Chemistry: C2 Alkyl Fragment Reactions and Probe Molecule Interactions

The thermally induced reaction of chlorinated ethanes on the nearlystoichiometric α-Cr2O3 (1012) surface results in the formation of gas phase hydrocarbons including ethylene, ethane, acetylene, 2-butene, 2-butyne and dihydrogen, and deposition of surface chlorine adatoms. No surface carbon or combustion products are observed in any reactions indicating no thermally induced C-C bond cleavage occurs and surface lattice oxygen is not incorporated into surface intermediates. A combination of photoemission and Auger electron spectroscopies indicates the surface reactions of the chlorinated ethanes proceed via C-Cl bond cleavage to form surface chlorine adatoms and surface C2-alkyl hydrocarbon fragments (i.e. ethyl, ethylidene and ethylidyne). Temperature programmed desorption studies indicate that both ethyl and ethylidene intermediates are selective towards ethylene. However, ethylidyne is more selective towards acetylene, but also produces ethylene in significant quantities. Chlorine adatom deposition leads to deactivation of surface Cr reaction centers by simple site blocking. The interaction of water with the nearly-stoichiometric α-Cr2O3 (0001) and (1012) surfaces is structure sensitive. Water is sensitive to the difference in coordination number of Cr surface cations between the two surfaces, and on the α-Cr2O3 (0001) surface, water has also demonstrated sensitivity to the degree of surface Cr cation reduction (and/or reduced coordination). These observations allowed for the development of a surface treatment recipe for the nearly-stoichiometric (0001) surface.