Paper SS+HC+NS+PS-TuM5
The Step Sites of Ultrathin ZnO Promote Methanol Oxidation to Formaldehyde

Tuesday, October 23, 2018, 9:20 am, Room 203C

We investigated the adsorption and oxidation of methanol on ultrathin ZnO layers supported on Au(111) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. In the TPRS experiments, we found that only molecular methanol-¹⁸O desorbed from the planar ZnO bilayer surface at T = 220 K and 260 K following adsorption of methanol-¹⁸O at T = 100 K, whereas a partial oxidation product, formaldehyde-¹⁸O (~95% selectivity), and a small amount of carbon dioxide (C¹⁶O¹⁸O) were produced at T = 580 K at the bilayer-trilayer step sites. Computational modeling based on the DFT calculations identified the adsorption configurations of methanol on the planar ZnO surface and at the step sites, as well as the reaction pathways to gaseous formaldehyde. The most stable adsorption configuration was found to be a methanol molecule adsorbed at the bilayer-trilayer step sites with its C-O axis parallel to the upper terrace edge, forming a bond between its O atom and a Zn site on the lower terrace, and also a hydrogen bond between its H atom in the OH group and a lattice O anion at the upper terrace edge. Starting from the most stable adsorption configuration at the step sites, formation of gaseous formaldehyde was shown to take place preferentially via a methoxy ( CH₃O(ad)) intermediate, following the pathways CH₃OH(ad) → CH₃O(ad) + H(ad) → CH₂O(g) + 2H(ad) with an overall barrier of 19.0 kcal/mol. Formation of CO₂ was kinetically hindered due to a much larger barrier of ~ 38 kcal/mol to produce a lattice O-bonded formaldehyde (H₂COO_lattice(ad)), the proposed precursor leading to CO₂. These computational results suggesting the preference to produce gaseous formaldehyde from methanol oxidation at the step sites agreed well with the high selectivity toward formaldehyde observed in the TPRS experiments.