Image of the Month

October 2016

Thermally Selective Formation of Subsurface Oxygen in Ag(111) and Consequent Surface Structure

Figure 1: STM images of Ag(111) after exposure to AO (atomic oxygen) at Tdep = 490 K. Exposure duration is labeled in the upper left corner of each image, and the scale bar is in the lower right corner. Panels (A) and (B) show that the p(4 X 5√3) domain was predominant after brief exposures, as were areas of clean Ag(111) with isolated O adatoms that were observed as black depressions, as shown in panel (B). Panels (C)−(F) show that, with increasing AO exposure, several domains coexisted until the surface became uniformly covered in the striped pattern after 300 and 600 s exposures. Imaging conditions for each image were as follows: (A) i = 280 pA, V = 1.0 V; (B) i = 300 pA, V = 800 mV; (C) I = 260 pA, V = 0.400 mV; (D) i = 200 pA, V = 800 mV; (E) i = 300 pA, V = 900 mV; and (F) i = 260 pA, V = 0.970 mV. (ACS Catal. 2016, 6, 4640−4646)

A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance. This is due to the coexistence of several different surface reconstructions on oxidized silver surfaces. A further complication is subsurface oxygen (Osub ). Osub  are O atoms absorbed into the near surface region of a metal, and are expected to alter the surface in terms of chemistry and structure; however, these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how Osub  is formed and how its presence alters the surface structure. Using a combination of surface science techniques to quantify Osub  formation and the resultant surface structure, we observed that once 0.1 ML of Osub  formed, the surface was dramatically, and uniformly, reconstructed to striped structures at the expense of all other surface structures. Furthermore, Osub  formation was hindered at temperatures above 500 K. The thermal dependence for Osub  formation suggests that, under the industrial catalytic conditions of 475− 500 K for the epoxidation of ethylene to ethylene oxide, Osub  would be present and is a factor in the subsequent reactivity of the catalysts. These findings point to the need for the incorporation of Osub  into catalytic models, as well as further theoretical investigation of the resultant structure observed in the presence of Osub. (ACS Catal. 2016, 6, 4640−4646)

Credits:
Jonathan Derouin,† Rachael G. Farber,† Marie E. Turano,† Erin V. Iski,‡ and Daniel R. Killelea*,†

(ACS Catal. 2016, 6, 4640−4646)

†Department of Chemistry & Biochemistry, Loyola University Chicago, 1068 W. Sheridan Rd., Chicago, Illinois 60660, United States

‡Department of Chemistry and Biochemistry, The University of Tulsa, 800 S. Tucker Dr., Tulsa, Oklahoma 74104, United States

Images and data graciously provided by Dan Killelea, Loyola University Chicago, Chicago, Illinois.

Microscope:
RHK PanScan Freedom Microscope

Control System:
RHK R9 Control System