In situ STM imaging of Cu2O/Cu(111) reduction under near ambient pressure CO

Cu-based catalysts are involved in crucial industrial reactions such as methanol formation and the low temperature water-gas shift reaction under tens of atmospheres of pressure and temperatures in the range of 180 - 260ºC.1 However, traditionally, snap shots of Cu(111) morphology were taken under low pressure (< 10-6 mbar) and cryogenic temperature (< 90 K)2-3. In order to bridge the pressure and temperature gaps, as well as to investigate surface under reaction conditions, in-situ near ambient pressure experiments carried out at room temperature were performed by a NAP-STM. The knowledge of surface structural changes during reactions can provide insight into the reaction mechanism and help design better catalysts in the future

To start with a simple case, reduction of Cu2O/Cu(111) by CO was first studied. Cu2O/Cu(111) as prepared is shown in Figure  1A.  After 281s of dosing 10 mTorr CO on the surface, a metallic Cu phase appeared at the top step edge (Figure 1B). As CO consumes O from Cu2O, Cu fronts propagate towards the Cu2O region. A band of Cu hexagonal rings and 5-7 rings4-5 separates the two regions (Figure 1C to 1F). After 828s, the surface was reduced to Cu (Figure 1G). By measuring areas of Cu2O row structure, 5-7/hexagonal rings and metallic Cu in the in-situ images, the coverage change of the three species, or the reaction rate, is plotted in Figure 1H

We propose that Cu formed during Cu2O reduction has high mobility in CO and can diffuse rapidly to join a Cu step edge. The reaction intermediates are Cu hexagonal rings and 5-7 rings.

Figure 1. In-situ NAP-STM images of Cu2O/Cu(111) reduction by 10 mTorr CO after 46, 281, 374, 490, 603, 715 and 828 s (A-G). Plot H shows the area of the three species formed during CO reduction. Scale bar = 5 nm. Color corresponds to height. Tunneling conditions: 1.1 V, 0.83 nA

Atomic level images in the same place were continuously obtained as shown in Figure 2 with a schematic of a Cu2O compressed hexagonal ring structure (“44” row structure in STM). Over time, in the center of each Cu2O ring, a protrusion showed up. This is due to the removal of chemisorbed oxygen. In the black box, the Cu2O row structure relaxed to a hexagonal ring structure (Figure 2B), then rearranged to 5-7 ring structure (Figure 2C). In the lower part of the image a Cu island formed overtime. Two sets of images agree with each other

Figure 2. In-situ STM images of Cu2O reduction by 45 mTorr CO after 92, 184, 274 s (A to C). Scale bar = 2 nm. Color corresponds to height. Tunneling conditions: 0.9 V, 0.78 nA. Green circle serves as a land mark. Black box indicates formation of a 5-7 ring structure. White arrow points to formation of a Cu island. Black balls in schematic represent oxygen in Cu2O structure. Cu atoms and chemisorbed oxygen atoms are not shown.

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4.Yang, Fan; Choi, Y.; Liu, P.; Hrbek, J.;Rodriguez, J.A., Autocatalytic Reduction of a Cu2O-Cu(111) Surface by CO. J. Phys. Chem. C 2010, 114, 17042-17050.

5.Yang, F.; Choi, Y.; Liu, P.; Stacchiola, D.; Hrbek, J.; Rodriguez, J. A., Identification of 5-7 Defects in a Copper Oxide Surface. J. Am. Chem. Soc. 2011, 133 (30), 11474-11477.