Reactivity Of 3-hexyne On Oxygen Modified Ru(001) Surfaces: Observation Of Oxametallacycles By Rairs

摘要

The chemical behaviour of 3-hexyne on oxygen modified Ru(001) surfaces has been analysed under ultrahigh-vacuum, using reflection-absorption infrared spectroscopy (RAIRS). The effects of oxygen coverage, 3-hexyne exposure and adsorption temperature were studied. Two modified Ru(001) surfaces were prepared: Ru(001)-(2 × 2)-O and Ru(001)-(2 × 1)-O that correspond to oxygen coverages (θ_O) of 0.25 and 0.5 ML, respectively. The striking result is the direct bonding to an O atom when the modified surfaces are exposed to a very low dose (0.2 L) of 3-hexyne at low temperature (100 K). For 0_O = 0.25 ML, an unsaturated oxametallacycle [Ru-O-C(C_2H_5)=C(C_2H_5)-Ru] is proposed, identified by RAIRS for the first time, through the vC=C and vCO modes. Further decomposition at 110 K yields smaller oxygenated intermediates, such as acetyl [μ_3-η~2(C,O)-CH_3CO], co-adsorbed with a small amount of carbon monoxide and non-dissociated species. The temperature at which a fraction of molecules undergoes complete C-C and C-H bond breaking is thus much lower than on clean Ru(001). The ultimate decomposition product observed by RAIRS at 220 K is methylidyne [≡CH]. Another key observation was that the adsorption temperature is not determinant of the reaction route, contrarily to what occurs on clean Ru(001): even when 3- hexyne strikes the surface at a rather high temperature (220 K), the multiple bond does not break completely. For 0_O = 0.5 ML, a saturated oxametallacycle [Ru-O-CH(C_2H_5)-CH(C_2H_5)-Ru] is also proposed at 100 K, identified by the v_(as)O-C-C (at 1043 cm~(-1)) and v_sO-C-C (at 897 cm~(-1)) modes, showing that some decomposition with C-H bond breaking occurs. For this oxygen coverage, the reaction temperatures are lower, and the intermediate surface species are less stable.