Selective Oxidation of Methanol to Methyl Formate on Gold: The Role of Low-Coordinated Sites Revealed by Isothermal Pulsed Molecular Beam Experiments and AIMD Simulations

Abstract

To elucidate the role of low-coordinated sites in the partial methanol oxidation to methyl formate (MeFo), the isothermal reactivity of flat Au(111) and stepped Au(332) in pulsed molecular beam experiments was compared for a broad range of reaction conditions. Low-coordinated step sites were found to enhance MeFo selectivity, especially at low coverage conditions, as found at higher temperatures. The analysis of the transient kinetics provides evidence for the essential role of Au _x O _y phases for MeFo formation and the complex interplay of different oxygen species for the observed selectivity. Ab initio molecular dynamic simulations yielded microscopic insights in the formation of Au _x O _y phases on flat and stepped gold surfaces emphasizing the role of low-coordinated sites in their formation. Moreover, associated surface restructuring provides atomic-scale insights which align with the experimentally observed transient kinetics in MeFo formation.

Figure 1

MeFo selectivity at the end of the O-pulse in isothermal, pulsed MB experiments on flat Au(111) (open triangles) and stepped Au(332) (filled circles) as a function of the surface temperature. In these measurements, a methanol flux of 52.7 × 10¹³ s^–1 cm^–2 and an O-flux of 0.08 × 10¹³ s^–1 cm^–2 were applied.

Figure 2

MeFo selectivity at the end of the O-pulse in isothermal, pulsed MB experiments on (a) stepped Au(332) and (b) flat Au(111) as a function of the surface temperature for different methanol and O-fluxes: A (black): methanol flux of 52.7 × 10¹³ s^–1 cm^–2 and an O-flux of 0.08 × 10¹³ s^–1 cm^–2, B (blue): methanol flux of 52.7 × 10¹³ s^–1 cm^–2 and an O-flux of 0.4 × 10¹³ s^–1 cm^–2, C (green): methanol flux of 4.3 × 10¹³ s^–1 cm^–2 and an O-flux of 0.4 × 10¹³ s^–1 cm^–2.

Figure 3

Transient MeFo formation kinetics for Au(111) at a low temperature of 190 K for different O-fluxes: (a) 0.04 × 10¹³ s^–1 cm^–2, (b) 0.08 × 10¹³ s^–1 cm^–2, (c) 0.17 × 10¹³ s^–1 cm^–2, (d) 0.4 × 10¹³ s^–1 cm^–2. The methanol flux in all experiments was 52.7 × 10¹³ s^–1 cm^–2. The gray shading indicates the duration of the O-pulse.

Figure 4

Transient MeFo formation kinetics for Au(332) at a rather low O-flux of 0.08 × 10¹³ s^–1 cm^–2 as a function of surface temperature. A methanol flux of 52.7 × 10¹³ s^–1 cm^–2 was applied in these experiments. The traces are normalized to the MeFo rate at the end of the O-pulse to allow for a direct comparison of the transient kinetic behavior. The gray shading indicates the duration of the O-pulse.

Figure 5

Snapshots from AIMD simulations illustrating different degree of surface restructuring induced by adsorbed atomic oxygen (top) and root-mean-square displacements (RMSD) of the different types of surface atoms (bottom) on (a) flat Au(111) with Au adatoms (oxygen coverage: 0.19 ML) and (b) stepped Au(221) (oxygen coverage: 0.17 ML). Dashed white lines in the initial snapshot (0 ps) indicate the position of the step edges. Color coding: Au, yellow; O, red; Au adatoms [Au(111) surface], orange.