THE ADSORPTION AND REACTION OF 1,2-PROPANEDIOL ON AG(110) UNDER OXYGEN LEAN CONDITIONS

The adsorption and reaction of 1,2-propanediol CH3CH(OH)CH2OH(g) on clean and oxygen-activated Ag(110) has been studied using temperature-programmed reaction spectroscopy (TPRS), isotopic labelling experiments, and electron energy loss spectroscopy (EELS). On the clean surface the diol adsorbs reversibly, desorbing with an activation energy of approximately 16 kcal mol-1 at 265 K from a monolayer state and at 215 K from a multilayer state. Vibrational spectra at both 1,2-propanediol coverages are in good agreement with those for liquid 1,2-propanediol. On oxygen-activated Ag(110) 1,2-propanediol reacts with O(a) by 215 K to form adsorbed 1,2-propanedioxy OCH(CH3)CH2O(a) and water. 1,2-propanedioxy is stable up to 285 K, at which temperature C-H bond scission begins. A mixture of acetol CH3C(=O)CH2OH and, most probably, lactaldehyde CH3CH(OH)CH=O evolves between 285-380 K, with the maximum rate of formation occurring at 340 K. 1,2-propanediol, hydrogen, carbon dioxide, and water evolve at 350 K Results of isotopic labelling experiments indicate that pre-adsorbed O-18(a) atoms are incorporated only into the carbon dioxide and water products. These data suggest that oxygen atoms remain adsorbed to temperatures above 300 K at which they oxidize adsorbed 1,2-propanedioxy. Pyruvaldehyde CH3C(=O)CH=O and one or more m/q 74 products, tentatively identified as a mixture of acetol and lactaldehyde, evolve at 390 K Additional carbon dioxide is produced at 410-450 K and 550 K, and residual carbon due to incomplete combustion of 1,2-propanedioxy remains adsorbed after the surface is heated to 700 K. The evolution of a mixture of acetol and lactaldehyde, rather than only the more stable product, acetol, is attributed to the small difference in C-H bond strengths of the C-H bonds at carbon-1 (primary C-H) and carbon-2 (secondary C-H) coupled with the possible effect of steric hindrance due to the methyl substituent at carbon-2. The reaction of 1,2-propanediol with O(a) to yield 1,2-propanedioxy and the subsequent decomposition of this intermediate with heating emphasizes that O-H bond activation and C-H bond scission are general mechanisms for the partial oxidation of diols on Ag(110).