Polarization Modulation Infrared Reflection Absorption Spectroscopy of CO Adsorption on Co(0001) under a High-Pressure Regime

Abstract

The adsorption of CO on Co(0001) has been investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-RAIRS), which has been applied for the first time in a study of a model system for a heterogeneous catalyst. The CO/Co(0001) system was studied in the pressure range from 10 -10 to 600 mbar at temperatures between 300 and 550 K, showing the in situ potential of PM-RAIRS and the significant scope of this method for catalysis research. Linearly and bridge-bonded CO species could be distinguished on well-annealed surfaces. High-pressure RAIRS experiments done at room temperature were in agreement with previous low-energy electron diffraction (LEED) investigations in ultrahigh vacuum (UHV) at 100 K, 3,4 indicating a transition in the CO layer from a ( 3 × 3)R30°to a (2 3 × 2 3)R30°structure with increasing CO coverage. By comparison of well-annealed and Ar-sputtered (defective) surfaces, we could identify, at a high frequency of around 2080 cm -1 , a CO species attached to defect sites. It is shown that annealing at 450-490 K at 100 mbar of CO pressure leads to the creation of defects at the cobalt surface. The defects influence the structure of the CO overlayer. The nature of this "defect"-bound CO is discussed. Postreaction X-ray photoelectron spectroscopy (XPS) showed the development of surface carbide upon annealing in CO, which is in good agreement with the vanishing of the RAIRS signal of adsorbed CO at temperatures above 520 K.

Structural Modification of Cobalt Catalysts: Effect of Wetting Studied by X-Ray and Infrared Techniques

“…8). In agreement with recent studies of carbon monoxide adsorbed on cobalt single-crystals [24,25] the band could be due to cobalt metal atoms with low coordination (i.e. at defect sites or in small particles).…”

“…Figure 9 FTIR spectra of CO adsorbed on Co/TiO 2 (a), before and (b), after reduction, passivation wetting and drying. alumina, to the complexes of carbon monoxide with metal species in defect sites or on the surface of small particles [24,25].…”

A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer–Tropsch Catalyst

“…Moreover, a two times higher linear:bridge ratio of adsorbed CO is found for the NC catalyst, which might be ascribed to a higher fraction of edge and corner sites at the surfaces of the small cobalt particles in the NC sample than present at the larger particles of the AC sample. A similar effect has been observed for adsorption of CO on a defect rich Co (0001) surface, where a lower quantity of bridgebonded CO was found as compared to the amount present on an annealed surface with a low amount of defects [36].…”

Model studies in heterogeneous catalysis at the microscopic level: from the structure and composition of surfaces to reaction kinetics

“…Among the experimental methods which have been applied to obtain spectroscopic information at elevated pressure on model catalysts are for example sum-frequency generation (SFG, 33,34 and refs. therein), polarisation-modulation IR reflection absorption spectroscopy (PM-IRAS, 35,36 ) or high-pressure XPS (X-ray photoelectron spectroscopy, 37 ). A final experimental aspect is the developments of methods, which allow detailed investigations of the reaction kinetics on model catalysts under well-controlled conditions.…”