Specific Ethene Surface Activation on Silver Oxide Covered Ag{111} from the Interplay of STM Experiment and Theory

Abstract

High-resolution scanning tunneling microscopy (STM) images at 5 K, simultaneously resolving the molecular adsorbate and the honeycomb structure of the well-defined Ag[111]-p(4 x 4)+Ag(1.83)O substrate, assign the adsorption site for ethene on the silver oxide surface. Ethene molecules are exclusively adsorbed above a particular subset of Ag(delta)(+) sites in the hexagonal rings of the oxide. Extensive density functional theory (DFT) slab calculations confirm that this is the most stable site, with an adsorption energy of 0.4 eV (39 kJ mol(-1)). Adsorption is accompanied by a large deformation of the hexagonal oxide ring and a significant increase in the C-C bond length. STM image simulations provide qualitative agreement with the experimental images, and the molecular orientation is discussed with the help of simple molecular orbital arguments.

Oxidation of Ethylene on Oxygen Reconstructed Silver Surfaces

“…And, as with the Ag δ+ in the oxygen-induced reconstruction on the (110) surface, on the (111) surface this positive charge on Ag facilitates ethylene adsorption. The computed favored adsorption site, see Figure 3 state (1), agrees with previous STM measurements [11], though it should be noted that these measurements were originally interpreted with a since disproven 2D oxide-like model of the p(4x4) structure. While 13 our computed adsorption site on the Ag 6 triangle is in agreement with previous experiments, the interaction with the correct model of the p(4x4) reconstruction is weaker than what was predicted on the early oxide-like model [11].…”

“…The computed favored adsorption site, see Figure 3 state (1), agrees with previous STM measurements [11], though it should be noted that these measurements were originally interpreted with a since disproven 2D oxide-like model of the p(4x4) structure. While 13 our computed adsorption site on the Ag 6 triangle is in agreement with previous experiments, the interaction with the correct model of the p(4x4) reconstruction is weaker than what was predicted on the early oxide-like model [11]. In our case ethylene adsorption on the favored Ag δ+ site is 0.08 eV exothermic and is accompanied by a lengthening of the C-C bond to 1.34 Å, similar to the oxygen-reconstructed Ag(110) surface previously discussed.…”

“…For comparison, the adsorption energy on the disproven oxide-like model of the p(4x4) reconstruction was previously computed to be 0.39 eV and the C-C bond length 1.37 Å. We did not consider this model in our calculations and refer the interested reader to Reference [11].…”

“…Thermodynamic considerations are often used to rule out the involvement of Ag 2 O, as the bulk oxide is not stable under epoxidation conditions 9]. Instead, at the oxygen chemical potentials required for epoxidation surface reconstructions with Ag δ+ sites available for ethylene adsorption are formed [6,10,11]. Such structures have been observed directly by scanning tunneling microscopy (STM) [10,11] and the presence of chemically similar oxygen species under near ambient pressure conditions can also be inferred by in situ X-ray photoelectron and near edge X-ray absorption spectroscopies (XPS and NEXAFS) [12][13][14][15].…”

Combined in situ XPS and PTRMS study of ethylene epoxidation over silver

“…The preferential adsorption of ethylene on Ag δ+ ions has been recently suggested by Bocquet et al [51], who studied sequential adsorption of oxygen and ethylene on Ag(111) using STM and DFT calculations. The correlation of the ionic silver and the nucleophilic oxygen was also observed in our XPS experiments (see Figs.…”