Abstract
Carbon monoxide participates in many copper-catalyzed reactions, which makes CO-induced structural changes of Cu catalysts key for important industrial processes. We have studied the interaction of carbon monoxide with the Cu(100) single crystal termination at 120, 200, and 300 K by means of low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), and density functional theory (DFT) calculations. The absorption band of CO (2082–2112 cm–1) at elevated gas pressure (up to 5 mbar) and at 200/300 K was found at a higher wavenumber than the characteristic band of the c(2 × 2)CO structure and was consistent with CO adsorbed on low-coordinated Cu atoms. The combined PM-IRAS/DFT analysis revealed that exposure to CO induced surface roughening through the formation of Cu adatoms and clusters on the (100) terraces. The roughened surface seemed surprisingly active for CO dissociation, which indicates its unique catalytic properties.
Highlights
Both carbon monoxide and copper are key players in the industrial synthesis of methanol. The latter is produced from a mixture of CO, CO2, and H2, typically over a Cu/ ZnO/Al2O3 catalyst at 510−530 K and 50−100 bar, in a process developed by ICI in 1966.1 It is commonly agreed on that CO2 is the main carbon source of methanol[2] even though methanol synthesis from CO on Cu/ZnO is feasible.[3]
Cu in UHV at cryogenic temperature has been well studied by various experimental techniques, such as infrared reflection absorption spectroscopy (IRAS),[16−19] photoemission spectroscopy (PES),[20−23] low-energy electron diffraction (LEED),[24−26] pump−probe,[27−29] and high-resolution electron energy loss spectroscopy (HREELS).[26]
After cleaning the Cu single crystal prior to each experiment, a LEED pattern characteristic of the (100) surface was observed (Figure 1a), and no contaminants were detected by X-ray photoelectron spectroscopy (XPS) (i.e., in the O 1s and C 1s binding energy (BE) regions, Figure S1)
Summary
Both carbon monoxide and copper are key players in the industrial synthesis of methanol. The first feature was assigned to CO adsorbed on top sites, whereas the second one was ascribed to CO adsorbed at stepped sites In these early studies, the adsorbate-induced reconstruction of the Cu substrate[32] was not considered, despite being known for other metals. CO induces restructuring of stepped platinum surfaces[33] and supported metal nanoparticles, as shown by electron microscopy,[34] infrared spectroscopy,[35] or scanning tunneling microscopy (STM).[36] Very recent high-pressure (HP-) STM and near ambient pressure Xray photoelectron spectroscopy (NAP-XPS) investigations of Cu(111),[37] Cu(110),[38] and Cu(100)[39] single crystals by Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift. Apart from Cu cluster formation, which may be important for methanol synthesis by maintaining a rough active surface and counteracting sintering, the modified surface exhibited unexpected activity for CO dissociation
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