CHEMISORPTION AND THERMAL-DECOMPOSITION OF ACETIC-ACID ON PT(111)

Acetic acid (CH3COOH, CH3COOD and 13CD3COOH(D)) adsorption and decomposition on Pt(111) has been studied by HREELS and TDS. At low acetic acid coverages (theta < 0.3), dissociative adsorption occurs forming a surface acetate species with an eta-2(O,O)-CH3COO configuration. Further dissociative adsorption as CO(a), O(a) and CH(x)(a) with x = 1-2 is also observed (theta approximately 0.2 or less). Molecular adsorption occurs at moderate acetic acid dosages (theta approximately 0.5 or above) with a finger print peak at approximately 932 cm(1) which is a well documented gamma-(OH) mode of the CH3COOH dimer. Hydrogen bonding between neighboring acetic acid molecules is observed at submonolayer coverages which is responsible for the stabilization of the acetic acid hydroxyl group. We estimate the hydrogen bond energy for acetic acid dimer on Pt(111) to be approximately 7.3 kcal/mole and the R(O...O) distance to be approximately 2.68 angstrom. Annealing the acetic acid/Pt(111) system results in the thermal decomposition yielding H2 and CO. Three hydrogen desorption peaks are observed at 230, 370 and 498K which are assigned as from the cracking of the parent molecules, combination of surface H(a) species and the dissociation of CH(x)(x = 1 approximately 2) species, respectively. With the C-13 labeled acetic acid molecules the observed three carbon monoxide desorption peaks at 400-500K 520-550K and 700-800K are assigned as from the carboxyl group decomposition, the combination of neighboring C(a) and O(a) and the combination of C(a) and O(a) with large surface diffusion barrier.