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Observations of the CO Adsorption Site on CoOx Nanoislands and its Active Phase in Near Ambient Pressures

  • Eoghan Rattigen (iNANO, Aarhus University, Denmark)
Date
Tuesday 31 March 2020
Time
Address
Hotel NH Noordwijk Conference Centre Leeuwenhorst

Eoghan Rattigana, Jonathan Rodríguez Fernándeza, Jeppe Vang Lauritsena
a Interdisciplinary Nanoscience Centre (iNano), Gustav Wieds Vej 14, Aarhus, Denmark

Cobalt oxide has been extensively researched as a low temperature CO oxidation catalyst, with the hope over overcoming the ‘cold start’ problem in the automotive industry[1]. Previous studies have shown that a noble metal support enhances the catalytic properties[2]. As such we prepared a model catalyst of CoO bilayers on a Pt(111). Previous studies have shown that a bilayer structure of the cobalt oxide is a good low temperature catalyst in UHV, outperforming a more oxidised trilayer structure, gaining significant enhancement from the metal support[3]. We will show the active phase of the cobalt oxide in operando conditions in the presence of mbar amounts of reactant gases as we increased the temperature from room temperature to operation temperature. Tracking the oxidation state of the cobalt was observed using a combination of NAP-XPS and NEXAFS performed at both CIRCE beamline (ALBA) and HIPPIE beamline (MAX IV) allowed us to determine the active phase of the CoOx.
The adsorption sites of CO on the CoO bilayer were observed using NAP-STM in a mbar CO atmosphere. These adsorption sites are thought to be on top and bridge sites on the basal plane of the CoO. These produce two distinct phases as seen in Figure 1(a), having different intermolecular separation. Region (i) has a smaller spacing similar to that of the lattice oxygen spacing in the CoO bilayer, believed to be an on top adsorption site. Region (ii) is assigned to be the CO adsorbed in the bridge site of the bilayer, coordinated to 2 lattice oxygen atoms. These carbonate species are believed to be key intermediates in the CO oxidation. This is corroborated by the NAP-XPS data, an example of which is shown in in Figure 1(b), where the two adsorbed carbonate species are shown (i&ii) and in line with literature for CO on metal oxides and CoCO3. The desorption of the carbonate species as seen when the temperature is raised to 150°C is accompanied by a notable increase in the CO2 signal measured with QMS, confirming this the importance of this intermediate step.

Figure 1 (a) shows a NAP-STM image of a CoO bilayer grown on a Pt(111) substrate (black) in 1mbar CO, with two distinct adsorption sites: i believed to be CO adsorbing on the top sites of the CoO and ii believed to be the CO adsorbed in bridge sites creating a CoCO3 carbonate species; (b) shows NAP-XPS spectra of the C1s region for a CoO bilayer grown on a Pt(111) substrate. The peaks, marked i and ii, correspond to the same regions in (a) the Co-O-CO and CoCO3 adsorption sites. The on top and bridge site for Co adsorbed on the Pt substrate are shown in blue.

References

  1. R. C. Rijkeboer, Catalysts on cars − practical experience. Catal. Today. 11, 141–150 (1991).
  2. A. Törncrona, M. Skoglundh, P. Thormählen, E. Fridell, E. Jobson, Low temperature catalytic activity of cobalt oxide and ceria promoted Pt and Pd: -influence of pretreatment and gas composition. Appl. Catal. B Environ. 14, 131–145 (1997).
  3. J. Fester, Z. Sun, J. Rodríguez-Fernández, J. V Lauritsen, Structure of CoO x Thin Films on Pt(111) in Oxidation of CO. J. Phys. Chem. C. 123, 17407–17415 (2019).
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