C-C AND C-H BOND ACTIVATION OF 1,2-PROPANEDIOXY BY ATOMIC OXYGEN ON AG(110) - EFFECTS OF COADSORBED OXYGEN ON REACTION-MECHANISM

The stability of adsorbed 1,2-propanedioxy OCH(CH3)CH2O, generated via 0-H bond activation of 1,2-propanediol by oxygen adatoms on Ag(110), has been shown to be sensitive to the relative concentrations of 1,2-propanedioxy and O(a). When the concentration of O(a) is sufficiently large, OCH(CH3)CH2O(a) is formed via 0-H bond activation upon the adsorption of 1,2-propanediol at 175 K. C-H bond activation, nucleophilic attack by O(a) and C-C bond scission subsequently occur to yield formaldehyde, water, formate and acetate by 275 K. Acetol CH3C(=O)CH2OH evolves at 335 K, driven from the surface either by the onset of the conversion of CH3COO(a) to HCOO(a) and CO2(g) or by through-surface interactions. CO2(g), H2O(g), acetol, lactaldehyde CH3CH(OH)CH=O and 1,2-propanediol evolve at 360 K. Production of pyruvaldehyde CH3C(=O)CH=O, acetol, and lactaldehyde occurs at 415 K and is accompanied by the evolution of additional CO2(g) and H2O(g) due to formate decomposition in the presence of O(a)(OH(a)). No H-2(g) evolves. Residual acetate decomposes to yield CO2(g), CH3COOH(g), and CH2=C=O(g) in the absence of O(a) near 580-620 K. The evolution of H2CO(g) and H2O(g) at 275 K and the production of acetol at 335 K suggest that initial C-H bond activation occurs preferentially at the central carbon of 1,2-propanedioxy. The production of lactaldehyde at both 360 and 415 K and pyruvaldehyde at 415 K indicates that with heating subsequent C-H bond activation occurs at carbon-1. Furthermore, these results demonstrate that 0-H and C-H bond activation and C-C bond scission are characteristic oxidation mechanisms for diols on oxygen-activated Ag(110).